I have a couple of huge textbooks being put to use to prevent escape at the moment, too.

You could make a secure enclosure for pretty cheap if you use a tub. Some people don't like to use tubs because they are not the most attractive way to house BPs, though. I personally house mine in tubs in a snake rack and never had any issues with escaping BPs until I dismantled part of my snake rack and put the lids on 2 tubs instead of a shelf above them, hence the necessary textbooks these days.