Look at the idea in the picture below. It’s five pillowcases connected, then stuffed with pillows.
I would recommend pillowcases that aren’t open on one end. Perhaps either a zipper closure or pillowcases that open in the bottom with a slit across the middle.
Funner idea 1
I don’t have time to try now as I’ll be traveling for a month soon, but try this idea.
Instead of buying pillowcases, or fabrics to make pillowcases, buy a curtain set you can lie on and be comfortable, with a big print rather than be limited to little patterns or generic pillowcases. Make sure the curtains will be long enough for your height, or your guests’ general height, of course.
The top layer can just be a curtain panel as it were. On the underside, you could make it in 2 pieces for each pillow case for a slit opening as if you were making the undersides of all those pillowcases. Join all pillowcase bottom lefts together, then all pillowcase bottom rights together, so you get two long bottom piece the length of the curtain panel at the top.
Put top to bottom good sizes together, serge all the way around as if one big long pillow.
Flip inside out and sew dense zig zag or other dense pattern stitches across the long mattress, where the pillowcases are joined at bottom. Then stuff the pillows in.
You’ll then have a long mattress with one continuous print, or one big picture if that were your print!
I’ll try this in April or May when I get back on my travels but I know it’ll work as I have tested it with paper.
Funner idea 2
Make each pillowcase out of a big print so that you have something like a panel on each. Panels are easy enough to find in fabric stores, but beach towels are also a great source for big print images on fabric! And imagine if you had Transformers on those images as you unfold the mattress, transforming it from one compact item to a long item before stuffing with pillows! 🙂
The pillows make lumps in the mattress, as you can see. Try to find pillows that either have less thickness, or are less round in shape to minimize the gap between pillows in the mattress. The former might be easier to find, and should be good enough for a decent cushion. The latter will definitely cost you more.