I had a very similar setup for a few years and loved it. I've since moved on to something a little bigger but this was a great overnight/weekend setup that was easy and cheap.
Forget the foam from the craft store... For how much you'll need at a thickness that will actually support your weight it will be over $100 easy (was there a few weeks ago pricing foam for another project). Head over the WalMart and pickup a mattress topper. I had a 4" memory foam one in my setup that I cut down to fit perfectly (about $80 for the one I had). This was SUPER comfy too, better than my bed at home.
Invest in a hi profile shell if you plan to sleep back there. I had a cab hi and while pretty, it sucked being inside. Sitting up involved having your head at 90*.
Build yourself a basic platform for storage. Here is some info about my setup I had and some lessons learned. http://www.clubfrontier.org/forums/...ruck-round-2-a-83733/index22.html#post1535369
Since your a short bed this might be of interest to you:
Mid-Size Truck Tent -
