I've run into the same problem. I build large models of great lakes ships - think about a 730ft real world length ship at 1/200 or 1/92 scale.... we are talking around 46" long model. I looked high and low for an acrylic case to protect/display my models in..... There is not a lot out there on the retail market, so I feel your frustration!
I ended up finding a local industrial supply business that sells sheets (4 ft x 8ft) and had them cut it into 12" long x 48" wide sections, and then I did the final trimming, etc. on my own, including the adhesive, and made the wood base as well. Cuts great with a table saw or even made blocks and used a circular saw. Also, the first time I built a case, I got lucky that this place had remnants for like $10.00 ea and had enough I just had to do a little trimming and glue them together, and I had a respectable case to use at home.
It's not perfect by any means ( as in not something I'd put in a museum ), but works fine for displaying at home and protecting my builds.
That being said, I am going to check into the 2 mentions above by Rick and and the one in Brampton, as I am in Windsor, Ontario