Piece of cake is right. Make it yourself. I will caution that if it's for children we recently had an incident where my 1 1/2 year old got stuck between her mattress and the frame because of a poor frame design (stupid hand-me-downs) that I haven't addressed yet. Don't make the gap for bedding too large. 1 inch should be more than sufficient as Steve indicated.
I've never understood not using a box spring, they're ridiculously cheap and make for a far more comfortable bed (in my opinion) but if you don't have one, definitely put some ply under the slats. Or even a buckie board like used for bunk beds.
I've never understood not using a box spring, they're ridiculously cheap and make for a far more comfortable bed (in my opinion) but if you don't have one, definitely put some ply under the slats. Or even a buckie board like used for bunk beds.