Very cool machine, but I can't say I have any specific ideas for a bed build-out. I'd say watch a bunch of YouTube to see how others have done it, especially various Aussie "Tray" beds as suggested above. Make a list of what your NEED and what you WANT to have with you. Are you sleeping in/on the bed? How long are you going out for? What climate and terrains will you be in? Etc. etc. then starting putting together a list of what needs to be included. With your rig I don't think weight will be much of an issue, but I'd also check into GVWR, axle/tire ratings etc. Again, I highly highly highly doubt you'd run into an issue with weight given the tandem rear axles, but who knows.
-TJ