[AcidGambit]

If you make a lid from a sheet of plastic it will warp and become useless in short order. Plastic can be used in lids if its not in a sheet but rather in closed cells like greenhouse panels. BUT it gets extra ugly that way. One of the main reasons to have a rimless tank is for the aesthetics. Glass is a better choice because it won't warp and can be used as a sheet.

If using glass you might want to put a hole in it somewhere so you can grab it easily. If you don't have a means of grabbing it, it will be very difficult to pick it up for feedings, and cleanings and eventually you will probably break it.

There are other options technically. I made a partial lid out of wood to keep my newts from climbing out.

Making a wooden lid is not super easy though so I can't exactly recommend it, but its technically an option.

Another thing to consider is why you are putting a lid on to begin with. You mention concern about evaporation. While full lids will slow down evaporation, partial lids will not do much/anything. Honestly the only reason to have a lid is to keep fish that jump from coming into the tank or to keep out curious cats. In either scenaio a screen or net lid can work very well. There are plenty of diy screen making lids/supplies around online and in stores.

Since you will almost certainly be doing big water changes every week, evaporation is probably not going to be much of an issue for you. Just some thoughts.

Can you give some more details on how you made the wooden cover? I'm assuming that you had to seal the wood to protect it from moisture. If so, what did you use? Did you add a lip on the inside to keep it secured on top of the tank?