Plumbing – Reverse osmosis water saddle doesn’t fit waste pipe

Here are some that appear to be fully-plastic, and/or plastic + stainless steel

Without knowing what particular fitting you need (1/4 compression to what?), hard to get more specific than that. Full plastic fittings are out there, and really are what you want, as RO & DI water do love to eat anything that will give them ions. My reading on the above fittings is that the stainless (which is as good a choice as you get in metal, though aluminum may also work well from what I recall of an old lab D.I.W. system) is not in the normally wetted path.

Here is another supplier's nylon/polypro fittings:

I'm assuming this is for storm/rain water runoff and now sewage waste. For a location that freezes in the winter I've seen the recommendation to run the above ground gutters to an air gap and then to the top of a catch basin (hopper) like yours. This allows the melting ice to run off if the ground below is still frozen. If drain is already below ground, or you don't experience significant freezing in your climate, then using the below ground ports should be fine. These side ports tend to be designed to easily knock out, either with a gentle push, or at most with a hammer and screwdriver.