I have had luck in an uncylced 5g bucket with sand some rock natural sea water and a bubbler for 3 days. I would have had it longer but I was leaving and couldn’t bring it home
Just keep in mind what a sand dollar is an what it eats.
Its an urchin. But it burrows and also filter feeds from the WC trapping particles. So an established sand bed is a must and supplemental feedings are probably needed. Unlike a motile urchin that scrapes rock for a living, the dollar scrapes the sand so to speak.
A while back I kept one for a few years. I only saw it when his under-sand wanderings brought him to the front glass panel of the tank. I think they need a pretty dirty sand-bed to thrive.