My proposal has been house arrest at Mar-A-Lago with a twist:

First of all, no visitors. That includes no members allowed to come stay at Mar-A-Lago. Not even Trump's family. Mar-A-Lago can refund any membership fees because Trump will be the only resident allowed besides Secret Service and security guards hired by the feds.

Secondly, Mar-A-Lago is taken over by the federal government. Trump could owe fines as well as prison time and this could help pay the US back. (This also means he can't charge the Secret Service to stay in his club.)

Third, Trump's movements are highly restricted. He's confined to a small guest bedroom (not his normal one) for most of the day. He's allowed out for meals and 1 hour of "exercise" a day. Obviously, smartphones/computers/social media are NOT allowed at all.

Finally, Mar-A-Lago gets a downgrade. Trump doesn't get a cushy bed, but whatever a standard prison bed would be. He doesn't get his normal meals, but whatever slop the prison food service cooks for him. I wouldn't expect them to make his stay imprisoned at Mar-A-Lago into 100% the same as a federal prison, but they could get it as close as possible. (Keep any luxury for the guards and Secret Service. They'll need to be dealing with Trump daily. They deserve some comfort.)