How big is the "cave"? A hide they will like and use will NOT be much larger than they are. Thdey will even usae one that is smaller then they are. For them to feel secure, they should be feeling contact with surroundings. If the hide is "too big", depending on hoiw much too big, you can add damp moss to fill in the gaps.
Each of my BRB's has a moss box. I only have two sizes, shoebox or sweaterbox. Each with one or two access holes and filled with damp peat moss and damp green moss. They all LOVE them and spend most of their time in them. Coming out to drink and hunt/explore. I know wehn they're getting hungry by seeing some out of the hides. Another benefit is they look fabulous, that damp moss does wonders for their skin.
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Thanks,
Dave Colling

www.rainbows-r-us-reptiles.com
0.1 Wife (WC and still very fiesty)
0.2 kids (CBB, a big part of our selective breeding program)
LOL, to many snakes to list, last count:
19.29 BRB
13.18 BCI
And those are only the breeders 
lots.lots.lots feeder mice and rats 