Uh, have you SEEN the inside of a "regular mattress"? Cutting a slit in it would destroy it, plus sticking your arm into would be like sticking your arm into a pile of chicken wire fencing.
One of those all-foam mattresses might work, though. Not sure.
- like xkcd suggests, the only really important slit is for the arms.
- couldn't this be simulated by taking a regular mattress and simply cutting a slit in the right place?
- has anyone tried doing that?