get some steel, weld some steel into a box, attach some steel to bed/frame, enjoy!
but seriously, look on pirate4x4 for some ideas. check out my personal favorite (and local to us washingtonians
)
http://www.nw-wheelers.com. Tell 'em Boonie sent ya.
they're a good group of guys that'tt help ya out and there are prolly a few in your area that'll offer a hand to hlep build one with you. If I were closer I'd definitly lend a hand.
What are you looking for? a 4x4 flat bed? utility style bed? you can make it out of tube or box tube. I prefer round DOM tube as i have a tube bender for such things
I have a few "preliminary" pics on here from when i was building my LUV truggy (scrapped it) of a tube bed i was building to get some ideas floating in your head.
A simple flat bed idea would be to get some angle iron (2"x2"x1/4") and make a frame with it and put pressure treated boards in it and simply attach it to your stock mounts somehow. It will take a little bit of ingenuity and fab skills.
Let me know if you have ANY questions. I'd be more than happy to hlep you out either on here or NWW.