Get a big clear plastic storage tub
( I seem to be having to remind people of these just lately !!!!! ) . The very biggest you can get your hands on. And use that as a temporary tank until you can re-home the fish. A Fluval 3 or 4 internal cannister filter will do for now, and
you'll need to do 50% water changes every week at the very least to keep him healthy.
You can add bunches of fake plants , rocks and decor for him as you would in a proper tank. But since it would be temporary until re-homing, don't bother with gravel. It's easier to keep clean with a bare base too.
When he's been re-homed you can simply empty the tub, put the spare filter to run in your community tank to keep it cycled, and keep the empty tub and it's decor in storage for a quarantine or hospital tank.