There is a place here in town that sells foam and supplies for upholstery work. I'll bet there is something like it near you. (?)
Anyway... I bought a slab of 5" thick regular yellow foam. They had different densities to choose from. I sat on the samples to help make up my mind. I think I went with a medium or firm density, I can't remember. I also bought, from the same place, some padding. It has a name but I can't remember what it was. It looked like a cotton ball but it was synthetic and it was in a big sheet. It was about 1"-2" thick. I used a huge carving knife (an electric carving knife works great) and cut the foam to size. I then cut the padding to the same size and layered it on top of the foam. I then grabbed an over size bed sheet and sewed a loop around the perimeter. I fished a drawstring through this loop / pocket, wrapped it over the foam and then drew it tight and tied it on the underside. The sheet does not need to be neat. I just use it to cover the foam and make it look nicer. All of my crappy sewing work gets hidden under the mattress. I throw this into the bunk and then make my bed as usual, tucking the sheets under the mattress. The sheets get changed as usual, but the drawstring thing almost never comes off. Sorry I'm so vague on the names and stuff. It was seven years ago. Any good upholstery shop or supply house will know what I'm talking about and will be able to help you.
I think itís the most comfortable bed ever and my girlfriend agrees. One thing to look out for though, foam is made from petroleum products and thus is very expensive right now.
Good luck and sweet dreams.