I just had to figure out the same thing, there are options for pre made lids you can buy but most are screened versions. I wanted a solid top for my 75 gallon to keep the humidity and my 6ft boa in. What I ended up doing is going to home Depot and getting some 1x2s cut to my tanks measurements, and after I went home and attached them together I decided that if I was going to stain it black to match the tank it would be better looking to just get some plywood to go on top instead of the acrylic I was considering. So I went and found a 4x2x1/8 and had it cut for me. After putting it all together I used some stain and about 4 good coats of helmsman outdoor poly sealant to make it waterproof. Here is a pic of it before I put the hinges and hasp on.