This is the simplest model: Wooden frame with the inside dimensions that are the same as the tank's outside dimensions, then just attach a mesh by any method that suits you. I used a stapler.
After a minute of thinking, I have a few more ideas including hinged lids and lids that lock on to the frame of the tank.
There is one thing that is pretty popular and that's to tip the tank onto its side and make a sliding or hinged glass door in the front with ventilation holes in the ends. I'll see if I can make a picture.
edit:
don't know if you can make anything out of this picture, but this the general idea of tipping the tank over and making the top into a front.
