comments:
nowhereman136 posted on r/vanlife2w
Cheapest option is to buy upholstery foam on Amazon. Buy as much as you need and cut it to size. It's not as comfortable as a mattress, but it's good enough. This is what I used for a few months before getting a real mattress for my van. Better quality foam, cut to size, will be more expensive. Larger pieces are harder to make and move, so they will be more expensive than 72x24 sections.