I can think of a couple of possibilities (some not that realistic).
1) If and when he's convicted the government should seize Mar a Largo. Build a wall and turn into a Trumper's Paradise. Change it from a resort to a prison, bring in whomever the government is using for food service, and House Trump, co conspirators and all those convicted in connection with the Jan 6 Capitol attack. Throw in anyone else convicted of similar crimes.
2) GITMO is still a possibility...
3) Camp David seems to be under used recently. The perfect place for the Trump Crime Family to spend their remaining days.
4) We've got a place nearby here. Camp Peary, also known as The Farm. I'm sure there is a small cabin there that can be used. Also a few people who have ways of finding exactly what Kompromat Vlad is using against him.