You should also make sure to use clamps or large rubber bands so that the snake cannot force the lid off and escape that way. Binder clips from an office supply store work well for this. The substrate that I use is Desert Blend Lizard Litter , which is crushed walnut shells. It is not too expensive, and because the sand boas are desert animals who produce dry waste it seldom needs to be replaced. Playground sand is also a reasonable substrate, but it tends to be more dusty.
Shredded aspen is safe, but some sand boas seem to prefer feeling the weight of a denser substrate.
If you use aspen you need more depth than with a denser substrate. This also applies to the regular Lizard Litter, which is made from the stems of the kenaf plant. Never use cedar shavings, which are toxic to reptiles, or pine shavings, which may be. Both Lizard Litters and shredded aspen are generally available at most stores which cater to herp needs. The cage needs heat at one end, most economically supplied from beneath.