This is me thinking out loud. Maybe it will help others, too.
I'm excited about the IKEA solution for our van, but I've been worrying about how to keep the slats in place. I just remembered the queen bed my daughter has stored at our place is from IKEA, so took a look it. Just in case you have never seen one in action, here goes. All IKEA bed frames larger than twin use one SKORVA center support beam (901.245.34) running lengthwise up the center. Two separate LUROY slat units (unique to each bed size) span cross-ways, each one resting on an outside bed rail and the SKORVA center beam. The bed rails hold the slats in place on the outside, and the slats butt end-to-end on top of the SKORVA center beam, so they can't move. The slats within each LUROY slat unit are ribboned together to maintain spacing. Each slat unit is stretched taught lengthwise by securing the end slats behind little white plasic nubs sticking up at the ends of the bed rails and center beam. Simple and very effective.
Many RV conversions turn the whole thing 90 degrees and run up to 3 SKORVA beams across the van, like the ones in this thread. Our floorplan calls for a fixed bed at the back of the van, at the height of the PM's horizontal rail, like those in this thread. My floorplan has a partial wall at the end of the bed toward the front of the van. So I can build a "bed rail" into it, replacing one of the SKORVA beams, and trapping the slats at that end.
I still need 2 SKORVA beams. I'll attach a vertical board to the back side of rear one to act as the "bed rail" slat-stop. I will no longer be able to collapse (telescope) that rear SKORVA beam, but it should still lift out of its brackets, in case I want to remove the bed later (hauling home applicances, etc). I'll remove the extra slats and position little white nubs, same as IKEA. The nubs need to be placed so they keep the slats butted end-to-end along the center beam. That should do it, eh?