Ive seen people use cinderblocks, not sure how they dont bust the bottom of the tank out, but some people can pull it off!
And also, they are usually more expensive in the fish section, but go into the reptile section of your fish store and look at water proof logs, I got one for my sisters plecos for 7$ it was covered in fake little ivy leaves, but they only log on the fish side was an ugly brown for 15$