This is not a chicken wing ! ^^Always a setback when come to SB a curvature profile on Gunpla , thinking of how to ensure the parts accuracy when mirrored left-right or front-rear, especially a 3D curvature profile.
So as for myself , pre-shape plaplate template as foundation still the best way for this matter.
Some examples of curvature profile scratch building from 1/48 Sinanju head Display .
A set of cut out 1mm plaplate . The small center piece will be the template /foundation of a protrude curvy profile.
Adding a pre-shape rib as guide for epoxy putty shaping and sanding .
Initial shape done.
For 2D round profile, measure the desire size and with the elastic characteristic of Polystyrene, just bend with hand to soften the plaplate and try to Over do the U bend , yes Over do . It's much easy to push it out later than outer force to hold it in position.
Have another plaplate with desired dimension & shape draw on it , as a guide for accuracy. Glued.
Same technique applied to another 2 pieces of U-bend plaplate on top of the first one.
Again , Putty added if it have to be a 3D curvature profile.
Again a pre-cut thunder shape plaplate as foundation then sculpt on epoxy putty .
I will put extra 2~3mm thickness epoxy to the designed profile , so the following process i will only need to scrap off the extra but need no more topping.
Trimming should be done while the epoxy putty are half harden ( after 1~2 hours curing, depend on how much water you add on when mixing the epoxy putty) As it's strong enough to hold the shape and stay in place , but also soft enough to slice with cutter.
Sanding with file only can be done after it's totally harden (after 4~5hrs)
As usual, sanding with 400 sanding sponge on curvy profile , flat & sharp edges will need sand paper/sanding stick.
Basically , DONE :D