My matamata was always trying to hide under her filter (fluval 4) and usually ended up dislodging it. So we took a plastic litterbox, cut one side out, put holes in the top, and weighed it down with plant weights to make a cave. She crawled right in and has been sitting in it with her head poking out ever since. I never really though of a 'hidebox' for a turtle before, but she really seems content and is even eating more. (More fish! MORE!)
I can tell she's happy 'cause she's smiling.

