any plastic cave will do. clay pots are cheap, your own pile of rocks, cinder blocks. they just need a hole they can fit in and im sure they'll find it homey. keeping in mind males can get up to a foot long and females about 7-8in. i have had the problem of store bought gts being quite shy and only coming out of their cave to eat. with my breeding pair of terrors i had to pratically remove all cover to get them to mate. you may find yourself spending a lot of time making them a nice home, and then never see them again
I had a similar experience...my RD likes a cave, but when we had a cave for her (made out of paving stones to be large enough), she never came out, just defended her cave. So we took it down and she has become a lot more outgoing.