actually pvc and eggrate will probaby work.If you want to go cheap, you can construct a "stand" from eggcrate and 1 1/2" or 2" PVC pipe, zip-tied together. I'm assuming the "heavy" part is a rock? If you place the PVC pipe correctly, it shouldn't be an issue unless you need a crane to hoist it into the tank.
Another option would be to buy a plastic shelf unit at HD and cut it to fit. That would leave you with the question of what to do with the other three shelves though.