This guy completely re-did his bed into a 'Trayback', but added a rack & canopy (the canopy is a recollection ... forgive me if it's actually an embellishment, it's been a while since I read that thread).
http://www.expeditionportal.com/forum/threads/65630-2006-Tacoma-tray-back-100K-mile-re-do ... I think it's a bit farther than you may want to go with yours (and a bit farther than you *I* would go, given the condition your truck is in); but it may give you some more ideas to noodle on.
The other thing you could do, since you're already talking to an upholstery shop, is have them just make some snap-on panels to close out a contractor rack ... assuming you can find a contractor rack to fit the 720. Might be tough to make it fully waterproof, but I bet they could get close ... and it'd be unique!