Very nice!
I too spent $$ trying to make one for my 93 cube out of thin glass (failed, break easily), then 1/4" acrylic (failed, warped). Acrylic sandwiched with glass (failed, silicone don't stick well to glass).
Now, I found a couple of the old aquarium glass lids, work well. The only down side is to remove multiple pieces of glass when cleaning.
Also, I have a 1/2" acrylic lid (20"x20") on my rodi reservoir. The lid also have a 1" acrylic support across the middle. However, lid is slightly warped from water moisture.
However, looking back at your design, you have lots of slots on it. This might actually work better as it might not retain as much moisture in it. Good luck. Let me know how it work out down the road.