You can also order a nice mattress in a box from walmart online for not alot O money
. I've had an 8" thick one for 4 years now. Just use a bread knife or electric
carving knife to cut the foam to shape.
You can get fancy by making a pattern with a plastic drop cloth for the base and use a square and tape to measure any angled side offset at the height of the mattress. I'll mark every 2 feet or so and an inch offset to the sides. Mark the underside of the foam with the base pattern. Then move the pattern to the top and mark the offset distance. Then cut at the angles indicated between the lower and upper marks. A little tricky, but not too bad.
Then use the same pattern to make patterns for the top and bottom mattress cover, with boxing (sides) cut for the angled distance. Add an inch to the boxing and 1/2" to top and bottom for seam offset. Then sew the material together add a zipper to one long side on the bottom and you have your very own custom mattress for a wee bit of cash.