Julian is right, it's about the only good way to do it. How thick is the plexi you bought? NVM, reading > me. Cut the window in the enclosure 1" smaller all the way around, so since your plexi is 12"x24" then cut out 10"x22". Then cut out a frame that's with a hole the same size as the plexi but 1" wide all the way around (outside size 14"x26" with a hole 12"x24"). Next cut out another frame that is 14"x26" (outside dimensions) with a hole that is only 1/2" smaller than the frame (11"x25"). That will sandwich the plexi into the enclosure, be sure to use glue on the frame's like you did the enclosure and seal everywhere the plexi touches MDF with silicone. That will get the plexi mounted in there tight and look good from the outside because the inner frame is a little smaller than the window in the enclosure itself it will be difficult to see the frame. I used a router to step cut the plexi and MDF frame on mine so it's a little different, but this is how mine is done. Yours will just have two separate pieces of MDF instead of one, unless you have access to a router, then I could help walk you through doing it that way as well. Here's how my sandwich was done. And here's what it looked like when done. and Hope that's some help.