Hancocks is going out of business, JoAnn's foam is too expensive even with a coupon.
Better to order online or buy foam mattress from Walmart or Wayfair or Amazon. Don't buy too stiff a foam (seat cushion foam) or it will be too hard to sleep on even for a child.
If you have narrow bunks, can buy a full size piece and cut in half. If no electric carving knife, use a serrated knife and use it like a saw. If you can support the foam on both sides of the cut, it will cut much easier. You can always buy two egg crates, lay the convuloted sides together to make a thicker foam mattress. Wrap a cheap mattress pad around, then use a fitted knit sheet. Knit sheets are stretchy and will stay in place better than a regular sheet.
2016 Wildwood 26TBSS
2012 F150 Screw XLT
4 leggers Spudz and Halley