Keeping a tub dry???

[Pareeeee]

I'm getting rid of my huge oak entertainment unit that I've been using as a critter cage stand now for a few years. It's too big for our little apartment so I bought a shelving unit.

Boaz, my Rosy Boa is currently in a glass terrarium which stays nice and dry. I will be moving him to a tub once I sell the entertainment unit.

Now, Rosies need very low humidity. How do you keep it down in a tub? Lots of holes is a given, but is there anything else I can do? Are there certain things I can put in there that will 'absorb' humidity?

Thanks!