Glass tops that you purchase from you local LFS will do for that size. Once they get a bit bigger, you can either get large rocks or heavy books to put on top, or get thicker, heavier glass/acrylic from a local glass store.
i would normally suggest searching but in this case how about simply reading the top 5 most recent threads? http://www.monsterfishkeepers.com/forums/showthread.php?t=198776
2 pages of peoples recommendations on lids.. but to summarize, most people do acrylic or glass..