Haven't been on the forum in a couple of years, but, I have had some experience with this situation.

I have not used exterior forms, but, cut out parts and free'handed them together.. That was the way back then.
I had some bowing in a couple of places, and after talking to several folks from the forum, it was decided that I possibly tightened the stitches ( copper wires ) in a sequence that pulled parts in the wrong order and caused the warps/bends.

I loosened the wires on the whole hull, enough for it to be adjustable, then started again. I tightened the ends first so they would line up properly, then, slowly, went over the boat tighten each wire a half turn or so, then back over it again, for another half turn and watching the joints the whole time. It worked and all came together as advertised.
Ocassionally I would press parts that wanted to move around, while I was twisting the stitches, but, taking it slowly and in sequence seemed to help.

Also, it will only work if your parts have been cut out EXACTLY as laid out on the plans, whether by hand or by machine. If they vary at all, there are going to be issues with alignment, and if you try to force alignment, you will have parts not fitting elsewhere, which in turn causes bows and the like.

Hope this helps!

Rehd