Using the original pieces and this new size, you can now insert a 18"x24" piece of plexiglass without cutting.

To get to 18" use: 2 corners, 1 six inch side, and 1 - 214mm side. 

To get to 24" use: 2 corners, 2 six inch sides, and 1 - 214mm side.

My first enclosure had an 18"x18" front door and 18"x24" sides and top.  I needed to make it larger when I added the MMU3 to make room for the buffer. (Good thing this is modular!) So my latest enclusure, now has, 18"x24" door and all sides. The top is 24"x24"."  Very happy with the enclosure.