I think there are a number of issues that go into prison design. The glib answer is, "whatever produces the best outcomes," but I understand that leaving it at that is profoundly unsatisfying. I don't have the background in the domain to give a detailed answer, but I have some thoughts about things worth considering. I generally take "unpleasant" to mean strongly "not liked" at the time. There is, however, a distinction between liking and wanting, in terms of how our brains deal with these things. For deterrence, we want the situation to be "not wanted" - how much people dislike being in jail while actually in jail is irrelevant. It is also worth noting that both perceived degree of punishment and perceived likelihood of punishment matter.