There is no legal reason that I am aware of that Trump cannot be thrown into a jail cell. There are many practical impediments to that, of course, mainly connected to his absolute right to a secret service detail.
Surely it should be far easier for the secret service to protect a person who is confined to a secure facility?
Obviously it wouldn't be a particularly popular detail for whoever draws that particular duty, but standing around in a jailhouse drinking coffee isn't the worst duty a secret service officer could be assigned, and it's not like the individual officers would be in jail - like the jail wardens (and unlike their charge), they would get to go home at each shift rotation.
Random snipers and opportunistic assassins should be far less of a threat to someone who is in jail, than they would be to him if he was walking around freely in the wider community.
I think that would be extremely hard duty and I also think that US prisons are not equipped to provide adequate security to the run of the mill criminals, much less someone like Trump. Maybe prisons in Australia or elsewhere in the world are better than the ones in the US but here, they ain't so great, except perhaps the ones for white collar criminals--minimum security. I find that shameful but I'm trying to be realistic.
I think that Trump would pose a security risk wherever he was confined and would also be at significant risk, no matter where he is confined. Being confined in one of his properties, without the amenities he's accustomed to would be an extreme improvement over the imprisonment of most or all criminals so sentenced in the US. Frankly, better than most of them had access to as free persons. I think that having Trump killed in prison, as would be a reasonably predictable outcome if he were imprisoned in any of the US prisons would cause serious political issues for many years to come. I also think that in anything other than solitary confinement, Trump would also pose a significant risk to the security of the US and the world. It's not worth the risk, frankly.
My personal belief is that there is no sentence for Trump that would adequately punish him for his crimes and he is beyond rehabilitation. I don't believe in the death penalty but I think that Trump likely merits it.
The idea that he be confined in one of his properties without access to staff to do his bidding, cell phones, internet, Fox News and having his mail and contact with the outside world severely limited is a reasonable compromise. I would find it hellish, myself, to have to spend time in one of Trump's properties. But under such circumstances, his safety could be reasonably well assured as could the safety of the world from Mr. Trump. I mean, sure, I'd like to go all James Bond/Hunger Games/whatever and have some fail safes built in so that if he were to somehow manage to be illicitly sprung from his 'home/jail, he would be blown to smithereens but I try to maintain reasonable connection to reality.