i just used tupperware that could fit my pump that i had on hand, it works fine, i just leave it at an angle so i can tell when i need to add water, unconventional and no the best, but it works
I did exactly the same thing. You can buy pieces of glass cut to size at many local hardware stores. You do NOT want the window glass (not thick enough). I placed them in the tank where I wanted, fixed in place with silicone and the 1 piece I wanted to elevate I used a 2x4 to hold it up while it dried.
I did the same thing with my 125g. I took my old 55g and turned it into the sump for it. What I did is made two bubble traps and seperated the center compartment for my skimmer and a refugium.