First of all, I agree with the others that there is no harm done by plastic bags for sous vide. I have read a statement by the manufacturer that brand-name Ziploc bags don't release anything below 76°C. If you think how much a lawsuit could cost them if the information turned out to be wrong, I trust that they are telling the truth. For other brands, you may have to do some research about safe temperatures.
If you are still unconvinced, your choice of material is very limited. You say it must be airtight and waterproof; I'll add that it must be pliable, so it can cover an irregular steak shape perfectly, without leaving air pockets, and it must somehow be able to create a seal. Also, it must not release any harmful chemicals by itself.
About the only thing that fulfills all criteria would be a wax with a high melting point, like carnauba wax. You could paint the steak with the melted wax, or, probably better, you could soak a piece of gauze in the melted wax and wrap the steak in it, pressing out any air bubbles. Then let it cool and set before cooking. The downside: not only is the food grade wax difficult to source for private people, it also can end up costing quite a bit.
Another option would be silicone, like the one used for baking pans. Unlike plastic, food grade silicone does not contain any chemicals which could leak - the medical sector uses the same stuff for prostethics and implants; it is safe to have it inside your body, so it is definitely safe to cook your food in it. The problem is that you'll need a sealable bag made of silicone, and I don't know if anybody manufactures such bags.
You might consider very tightly wrapping the meat in a caul (not necessarily an amniotic caul, a peritoneum should do nicely) and binding it, but I don't know where you can get cauls. Maybe you can ask a butcher. Also, it probably won't be 100% watertight (but still enough to keep the tasty juices in the steak where they belong, instead of having them flow out into the water).
If you are willing to relax your rules a bit, you can solve the problem much easier. I think you will agree that whatever hypothetical substance might leak from a plastic bag, it cannot travel far through a solid medium. So a steak wrapped in something protective and then sealed in a plastic bag should be safe - you just discard both the plastic bag and the presumably contaminated protective substance. Yes, it is possible that meat juices that have come in contact with the plastic bag end up on the steak, but the possible contamination should be hundreds of times less than if direct contact is allowed between the plastic and the meat.
If you can live with this option, the usual insulators used in the kitchen should do. They are plant leaves - I'd use grape leaves, but you can use practically anything that is big enough - and batters, like tempura batter (breading leaves an irregularly shaped surface, so I won't consider it here, you'll end up with lots of miniature air pockets). If using a batter, you will want to first set it in a pan with very hot oil, just hold it there long enough to set the batter, but not enough to warm the meat on the inside above the sous vide temperature. Both of these options wont't function on their own, as they won't create a sealed waterproof barrier. But combined with a plastic bag and later discarded, they should be a good solution. The leaves will also add a nice taste of their own.
What you will be dealing with is called stratification. Given a reasonable volume of water the difference can be quite remarkable. A one metre height of water can stratify water from 20°C to 95°C as long as the water is not disturbed and heated gently, even if heated from the bottom
The simple solution is to regularly stir the water, say once every five minutes. This would be OK for items only requiring an hour in sous-vide, but for much longer this can get rather tedious
If you are targeting temperatures in the 40°C to 65°C range a small aquarium pump would suffice to stir the water. Arrange the intake tube nearly floating on top, and the output tube weighted on to the bottom. At a pinch an aquarium bubbler would help significantly too. Both these devices will cool the water somewhat, but a stove top heater should be able to keep up
Example: Using a small bubbler in a large, well-insulated chest (Rubbermaid cooler, Esky, chilly bin etc.) with around 10 l of water at 60°C, keeps the temperature within 1°C from top to bottom. Heat loss is around 1°C per 30 minutes. By adding about 0.5 l of 95°C water every 30 minutes it keeps the temperature constant over a few hours
To get precise temperature control within ±0.5°C which some sous-vide recipes recommend, you will need a PID controller. For general home use with temperature control or ±3°C you could get away with a stove top heater, with accurate power control and a thermometer control system. PID is not that hard, so you might as well add that to your controller too
Best Answer
On top of the stove the issue is boiling dry. At that point the temperature can rise almost unchecked. This will result in unpleasant fumes at best, flames at worst. Even in the latter case, because you should be using a fairly tight fitting lid, everything should be contained, but I wouldn't rely on that.
The oven is different. Once the pot is up to boiling point you can set an oven temperature of just over 100°C. In the unlikely event of boiling dry, the contents still won't exceed that temperature - no burning. Like you I assume an electric oven.
Using a sous vide circulator you'll actually struggle to get the inner pot hot enough - you generally want a gentle simmer for broth/stock. The outer pot will loose a lot to evaporation even if it never boils.
I'd have more confidence in the oven, but if you're shopping there's one more option - a slow cooker. Making stock is trivial: put in bones (and any veg etc.); pour over boiling water; leave on high for a few hours or low for up to a couple of days; strain. I normally use low overnight for chicken stock with onion, celery, & herbs. These are specifically designed to run for long periods unattended without boiling dry (as the power input is low and the lid fits tightly).