I ended up getting a glass canopy for a slightly deeper aquarium, (36"x18" - the last 1 & 1/2" being plastic) and had a local glass shop cut out 2 nice 1/4 circle pieces out of the corners to allow me to have access for my filters etc.
So far, so good! Of course, I needed to build a center support for it out of aluminium... had fun finding an epoxy that would be strong enough and work in a wet environment, but it's all working now
I'm currently doing a fish-less cycle - ammonia is dropping and nitrates climbing
Thanks for the suggestion!