With Rainbow Boas humidity is a crucial aspect of their husbandry. Anything under 70-80% is detrimental to their health and can lead to respiratory infections. Also, they tend to go for lower heat so shoot for under 85-87 degrees at the warmest spot. If you can get those two specifications taken care of in a glass aquarium setup (difficult but definitely possible) go for it, otherwise I know people definitely like the Iris Christmas Tree tubs for boa species.
When it comes to retaining humidity, cypress mulch is your friend. If you don't have one already, go to Walmart and get the $12 Acurite digital thermometer/hygrometer, it's an all-in-one unit that allows you to keep tabs on the environmental specs of the enclosure.