Chins don't use their house

I have just got two female chinchillas and I've noticed that they don't really sleep or use a wooden box that I got for them. Instead, they sit on top of it and go to sleep. Is this a perfectly normal thing to do or are they trying to tell me they don't like it? I don't know whether I should be trying to change it in some way or just leaving it alone because they've found a spot that they're happy to sit in and keep retuning to.

Any advice would be great :)