when my derasa was small, i placed him in a tupperware dish, inside the tank, with some flat pieces of rock rubble beneath him, and he attached to a small piece of it.
once he was attached, i would just sort of bury that slightly in the sand, to help keep him upright.
now that he's gotten larger, he isn't attached to that anymore, and is mainly kept in place by the weight of his shell pressing down in to the sand.
interestingly enough, when i picked him up to move him this past weekend it looked as if his byssal opening had largely closed up. so he may not even have the ability to attach anymore. i wish i had taken a closer look when he was younger, so see if there was a relative size change.