Acquire the outer shells? or Put the plexi in the bottom?
I got the shells from some disused ones at my gf's college, gosh, almost 3 years ago now?
As for the plexi bottom, I knew aquarium silicone doesn't bond very well, but it does seal. So, being very careful I cut plexi squares to fit inside the outer shell, then rounded the corners over with a belt sander. You have to go slow otherwise you'll round the corners over too much and be sol. After that, I measured the approx distance from the underside of the top, to the point where I wanted the bottom plexi to sit and cut 4 pieces of scrap wood to that length.
After that, I set the plexi inside, using the wood to support it, taped in place, and drilled 3 small holes straight through the sides of the shell into the edges of the plexi. I then used tapped the holes for some set screws (6/32 iirc) and gently screwed those in.
After that, I used some two part plastic epoxy that was supposedly clear (totally was not clear) to secure the bottom panel further into place.
After that not so clear epoxy, I cleaned the inside of the shell, and used aquarium silicone to seal it all up. First test leaked like a sieve. Apparently my holes were not drilled so straight and protruded into the inside of the shell.
So, resealed and water tested again, tested fine. The green one I painted, the clear one I did not. This is a relatively old idea though as someone posted a vid of it back in 2007 on youtube. Just merely took me this long to get off my arse and do it.