Tried out masonite packing strips, and they are ridiculously flimsy. Ripped up some old hardwood framing into 3/4" and 1" strips about 0.25" thick and cut the slots in the jigs.
Fitted the jigs to the 2 x 4s with hot glue and glue blocks, with non-standard spacing:
J5-J4 11.7" J5-J3 25.3" J5-J2 39 J5-J1 45.6
Fitted strips with hot glue and nails.
Cut top plates from 6mm MDF to match longerons, fiddled to fit near the firewall and fitted to the top of the jigs.
After a break decide the starboard side isn't quite curved right, so rip it off and re-profile
It has now hit 45C in the shed, so I take a break until the cool of evening, then fight the mozzies to start cutting aluminium flashing and foam.
Had long intended to fit the foam lengthways, but I chickened out, and did it as the plans say.