Home made lids(5/8" high grade fiberboard with 1'4" hardware cloth stapled underneath), encylcopedias, and bricks. No worries about someone moving my weights, as my reptile room is locked, and I have the only keys.
It doesnt have to look unorganized if you do them all the same(I know, I'm missing some encyclopedias in this pic)
My larger tanks use larger bricks. I have about 35 tanks set up this way, and I can make sure they are all secure in about 15 seconds by scanning the line-up of bricks.

This is an old pic, I have replaced of alot of the soda box hides with black plastic ones. I hope to start adding plastic plants soon too.