IANASSA, but in a secured facility, I don’t think Secret Service agents would need to be in Trump’s immediate presence 24/7.
I also seriously doubt he would have to be the only prisoner, either. I imagine a minimum security (i.e., “country club”) prison would do, with regular screenings of the other prisoners whenever Trump was in the general population for meals or exercise. Or perhaps he would have separate meal/exercise sessions with a select group of trustees, if mixing with the gen pop was considered too risky.
If the courts sentence him to prison or jail, the Secret Service will figure out a solution. They don’t get to say, “Sorry, no can do.”
And of course, assuming he loses, Biden could just rescind his SS protection.