Since it sounds like it might be difficult for you to get to a lumberyard to build your own solution, you might be better off using a pre-made product. AC-Safe, makes Universal Air Conditioner Supports that might be useful in your situation.
![enter image description here](https://i.stack.imgur.com/UmdmR.jpg)
The system requires you to screw the platform into the window sill (which may or may not be possible in your situation), then simply rests against the outside of the building to support the A\C unit.
I've also seen brackets that mount to the bottom of A\C unit itself, and offer support against the outside of the building (but I was unable to find an example online. I think Thermwell makes them).
If your soon-to-be-landlord is already making unreasonable demands, I would reconsider your choice to rent with them. Minor carpet staining from traffic is considered to be "wear and tear" in most states, as well as scuffing of the walls and other things that result from simply living in a space.
Unless you're a group of college kids (to which, I could understand creating some fear so that people don't go party-crazy), a "no stains period" policy is unlikely to be legally binding, however, that is a question for a lawyer. Personally, if those are the types of demands the landlord is making, I would go into your lease expecting a bill for every single thing that should be considered "wear and tear". I've often rejected apartments simply because the landlord has alluded to charging for repairs before I have even moved in.
As for a permanent wall-to-wall covering, I can't think of anything permanent that would would suffer the wear, be water tight, and especially fit a tight budget. If you are set on this rental, I would consider putting your money toward regular professional carpet cleanings and before moving out, and the purchase of some spot-treatment carpet cleaners for when the inevitable spill does occur.
Finally, if you can't trust your roommates or their guests to share/respect your concerns for the carpet, you're going to be disappointed.
-My 2 cents
Best Answer
It's totally normal. Carpet comes on rolls in widths from 11'6" to 13'. They need to cut off a chunk of that roll to fit the room.
It's the reason why if you're carpeting a 10x10' room, you can't buy 100sqft of carpet: they'll charge you for more like 120sqft (12' roll, 10' long).
It also depends on the layout of the room, and where they put seams (if necessary). Eg: an L-shaped room has a couple different ways to lay out the carpet, and depending the layout and where the seams are, you'll use a different amount.
A good installer will also consider traffic: if there are seams, you don't want them right in front of a door way, or preferably not running down the middle of a high traffic pathway. I think they also avoid putting seams within a foot or so of walls (so there isn't a narrow strip of carpet running along the wall). All of these things may end up using more carpet than a layout optimized just to minimize scrap, but will result in a much better looking and longer lasting install.
The other factor is in patterned carpets, they will use a bit more since the patterns need to line up. The bigger the pattern, the more is scrap needed to keep it in line.
The important bit is to keep the scrap carpet, unless you really don't need it, since you paid for it. Often there is a good size chunk, which can be used for many things: