Twin XL Gel Memory Foam Mattress Topper
Dorm mattresses are vinyl stretched over a spring board. Three inches of foam is the highest-impact thing you can add to a room you did not choose, and it rolls up small enough to survive move-out.

Twin XL, not Twin. The extra five inches is the whole point, and the number of people who discover this at ten at night on move-in day is remarkable.
Dorm mattresses are vinyl stretched over a spring board. Three inches of foam is the highest-impact thing you can add to a room you did not choose, and it rolls up small enough to survive move-out.
Raising the bed turns dead floor space into the only real storage a dorm gives you. Measure first: most housing contracts cap how high you are allowed to go, and they do check.
These go under the risers. Off-season clothes disappear into them in September and you genuinely forget they exist until the week you need a coat.