Hi Alan
I don't think I'd try to make a mold of the bodywork for this repair. What I would do is go inside the damaged bodywork with some racing tape to hold it all together and prevent resin from getting inside. Then button it all up and get a right angle 1/4" die grinder (Lowe's or Home Depot or HF). Cheaper is better - there's a lot of dust here. A 2" sanding disk kit and some 40 grit disks and have at the outside of the bodywork. You'll make a huge dusty mess but you can probably keep it under control if you're careful. Grind away paint and gelcoat about 1" from the cracked and damaged bodywork.
Take some fiberglass mat and tear it into about a dozen pieces about 1/4 the size of the total damage. I mix the resin according to instructions unless its hot (like here in FL) or cold (like in Canada). Then I adjust the amount of hardener accordingly. I have gone as low as 50% of the recommended ratio and as high as 130% depending on temperature. I'm talking polyester here. Epoxy requires more precise mixing. I take a 1" disposable chip brush, wet out the piece, add the first patch piece, wet it, add the second, overlapping the first patch by about 10% with the rest on the part, and continue til I'm done. If you see any low spots, tear off some more mat and add to bring it up to level. Don't worry about high spots yet. You're aiming to be a little proud of the target surface.
A tech tip is to under estimate the amount of resin needed and when you run out, just mix a new batch in the same pot. That way you don't over mix. If it starts to go off in the pot, mix the next batch with 25% less hardener.
Once finished, a heat gun can be used to encourage the repair to "kick" and harden.
Masking paper minimizes the resin getting where its not supposed to.
Once done, pull the side panel off, grind away at the inside until you are level with the original surface. The 40 grit is fine. If it's a big repair area, a 40 grit flap disk on an electric 4" right angle grinder works fast! A single layer of mat on the ground out area inside if you're nervous. You don't want to build up too much material.
Flip to the other side and grind to close to the target surface. Then switch to a hand sander with 60 grit and sand with an "x" pattern to keep things level.
A great body filler or fairing compound is Q Cells. They make a lightweight, water resistant fairing compound that sands more easily than Bondo. Mixes with your resin. Measure the resin, mix the Q Cells to get a peanut butter consistency paste, add hardener according to how much resin you measured. You probably will need about 1/2-1 oz and 2-4 tbs Q Cells. Apply with plastic spatula Never use Cabosil unless you want to sand for the rest of 2021. Sand with
80/150/220. Prime, fix pinholes with glazing putty, sand with 220, reprime, reglaze missed spots, etc. Finish sand with 320. Finish paint.
I clean up with acetone, use empty yogurt cups for mixing, and a clean used aluminum pie tin for body filler. They sell small measuring cups (8 oz size) at some paint stores and for sure at fiberglass stores.
Making a mold will be in my opinion twice the work and expense.
Good luck
John