The prisoner is in a crying obsidian box with water on the floor. If they try to break out, a guard will see or hear and trigger suicide immediately.
Outside Defence
Above the prison is a layer of sideways observers that detects someone mining in and trigger suicide. On the sides are walls of sand which triggers suicide if the sand falls down. Below the prison is a pearl glitch detector that, once again, triggers suicide if any string is updated. Many of the redstone lines are made of waterlogged powered rails, which is impossible to break with pistons or use chorus fruit to teleport into.
More Security
There are two stasis chambers for the two guards. A guard can at any time enter the cell via pearl glitchin through the wall. The guard can enter the visiting process by glitching through the floor. Visitors and prisoners access their beds through a block that moves once, making smuggling items impossible (the respawn system is a squilly bed glitch as well).
