I don't see the need for a physical prison when Trek seems to have life-suspension technologies in abundance. One could apparently just shove the person in a fancy box that turns off his lights for an indefinite length of time, without having to worry about bedsores or the like.
This would take away the torture aspect of imprisonment, because the person wouldn't feel anything (including confinement or the passage of time) when in stasis. But if torture were the point, I'm sure UFP technology could provide a wide range of options that would cause maximum distress with minimal physical consequences.
However, it doesn't seem as if the UFP believes in any sort of prison. People aren't kept locked in a cell for punishment: they are kept there for short periods of time because letting them move about freely would pose a risk. Long term imprisonment is extremely rarely mentioned; instead, it seems people are held against their will only for a standard length of time (six months) to facilitate psychological or medical treatment that removes their criminal urges. And even this holding seems to take place in rather benign conditions, if the New Zealand penal colony is any indication; only hopelessly violent cases would end up on Elba II.
(To be sure, Eddington was in long-term rehab in "Blaze of Glory", and Sisko found him in a holding cell. But that might have been just where he was being held just prior to his transfer to Sisko, rather than where he had spent the past months.)
For brief holding of a high-risk person, I don't see why privacy should be an issue. The whole point of holding that person is to constantly monitor what he's doing, after all, and to stop him from doing what he shouldn't. An aquarium-type holding cell would appear ideal for that purpose.
Timo Saloniemi