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

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.