I need new glasses - I read this as 'Jugs', and I wasn't thinking of the P-47 !
I did once see a rather expensive, plastic, or maybe wooden, adjustable jig-like thing advertised, but it's normally a case of building your own to suit the model and scale in question.
In the past, I've just used stiff card, sometimes formed into elongated boxes, which did the job.
I think usually plans are lofted and full size. So overlaying blocks and sticks for jigging are pretty easily made. A rib jig for a Christen Eagle is not difficult to make. I made a couple, one simple, one not so simple for the work platform for my Merlin's Magic build. Sooooo many jigs, so little space.