I might go ahead and frame the 20x24s, for presentation and protection during the show. I imagine their main purpose will be to attract people into the booth, where they might buy smaller prints. When I did a project on 20x24, I had trouble deciding on a frame size as well, so I just cut matboards that I think were 3-1/2 in. on the top (27 in. total width) and both sides, and 3-3/4 in. on the bottom (31-1/4 in. total height), and then bought sheets of non-glare Plexi and cut it to fit exactly. I backed the prints with 1/4 in. plywood, and used gaffer tape to make a 1/4 in. black border on the glass, and wrapped it around to the plywood to hold everything in position. This was because I was too cheap and broke to have 27x31-1/4 frames made, and had no time or desire to do it myself. The prints got accepted into a show, however, despite the frames made out of tape. (The juror told me he didn't even notice...it was the gallery manager who noticed.) However, there are less ghetto ways than gaffer tape to mount a print behind a frameless piece of glass...or you will be able to find pre-made frames that are 30x36, which is pretty close, though they look like fat borders to me. That is a matter of opinion, however.
As for your questions re: Plexi, FWIW, I used to work for a museum exhibit preparation company, and while actual glass is still the standard, many individuals (not museums) prefer Plexi, for various reasons, and we always stocked it. IMHO, it looks good and is easy to work at home. However, while it is cheaper than nice glass, good quality Plexi is still not cheap.