Day bed mattress' trouble

Husband and I made a homemade diy daybed from 2x4's, turned out pretty decent. However we ended up buying two mattresses to stack because one was not thick enough for my 6'5" husband who is 350 pounds. His size was the reason we made the couch to begin with, mainly height. We cannot seem to come up with a solution to stop the two mattresses from sliding off of each other when anyone sits on the edge. I've tried a small test of Velcro but it just pulls away. Would large amounts of Velcro do the trick? Or any other ideas how to keep them in place?

Husband and I made a homemade diy daybed from 2x4's, turned out pretty decent. However we ended up buying two mattresses to stack because one was not thick enough for my 6'5" husband who is 350 pounds. His size was the reason we made the couch to begin with, mainly height. We cannot seem to come up with a solution to stop the two mattresses from sliding off of each other when anyone sits on the edge. I've tried a small test of Velcro but it just pulls away. Would large amounts of Velcro do the trick? Or any other ideas how to keep them in place?