What's the best substrate for a sand boa?

What do you think the best substrate for an adult male sand boa is? I currently have mine in aspen shavings, but I don't really like it. Would something like Lizard Litter work better? Would sand be okay as long as I feed the snake in a separate tub without sand? What do you recommend?

Sand or aspen is fine.
I have kept Kenyans, rough scales, saharans, javelins and tartars on both substrates. The only difference is weight. I used seived bird sand, but play sand is also quite safe to use. You could also mix in some dry eco earth, as most sand boa's don't actually live in sand.
I also fed all of mine in their vivs. Just make sure that the fur is completely dry.

I have read that if you mix sand with a some Excavator clay in the right proportions it's supposed to hold a tunnel pretty well.

EDIT TO ADD: I kept mine on both chopped aspen (not shredded, the lizard litter kind) and sand and she liked both.