Thank you both for the comments. I had considered the crate idea but hoped I'd be able to get by without though as I get further into this it may be the only safe thing to do. I think the board (2x4 probably) idea is a good one.
My concerns were any bending moments caused by the 60 mph or so wind and whatever strain might be put on the hangers as they were pressed into the mattresses. In that regard I'd though of perhaps just cutting the mattress where they would make contact so no pressure would be put on them. Or possibly putting a piece of Styrofoam between the hanger and the edge of the slot.

So if anyone on Teamkitfox has done this please speak up with how you went about it.