I started looking into this when I saw that most of the decent gun vises were close to $100 for a hunk of molded plastic. I do quite a bit of woodworking so I figured I could build one of my own. I haven't started yet, but I drew up some plans on a free CAD program called Google Sketchup that works very well for this type of thing.
The design is essentially a very heavy base with a T-track in the top which will allow for a variety of different holders to slide in. It has some drawers in the front for tools and cleaning supplies. I'm thinking of removing the middle drawer and replacing it with a suitcase type handle so the whole thing can be picked up and carried with one hand.
With the t-track you can have the standard rests, or you can have a middle piece that slides in like the Tipton GunVise has for holding up a folded AR for cleaning. In addition, you could have a seperate rest meant for holding pistols or revolvers for cleaning and for lighter weight jobs, you could have a flat rest with a small actual vice bolted to it for holding parts for filing, etc.
I've included a quick drawing of a simple rest so you can see how it would attach. There is a 1/4" rail that runs in the track and you can see a hole where a 1/4 bolt will pass up through the rest to a knob. The rail along the bottom is there so I only need one knob to keep it from twisting. (Yes, there should be a notch on the bottom for the bolt head.)
It may be heavier than I like but if it is, maybe I can adapt it for a shooting bench rest. *shrug* I'm also thinking of maybe only have the drawers go halfway through and having drilled or routed out pockets in the back half for tools and/or pieces that may be used fairly often and/or tend to roll around.
I was also thinking of cutting a 3/8" dado in the back with a wider mouth before assembly so I have a 3' deep hole I can store my one piece Pro-Shot cleaning rod in. Just ideas I'm bouncing around in my head.