Pretty-much the same as is done with standard frame jigs, except rotated through 90 degrees. So - with a similar jig to that I'm currently using for commerical frames - elastic shockcord would hold the 'under'-top and bottom bars firmly against the sides of the box jig. Gravity would hold the first side onto those bars whilst the glue is setting - the initial set of which is relatively quick. There's nothing to stop you putting a light weight on top of that side to hold it in place, (say) a 12mm (1/2") nut. Repeat for as many frames as your jig will hold. Once set, flip the jig over and do the other sides. When each basic frame is glued-up, then attach the 'top'-top bar.
Or - whack a nail in - if you really want to ...
LJ
