It depends. When I made my fiberglass packs for the Rocket City Ghostbusters I made my own mold because I needed to make four of them and it would have taken way to long to build 4 packs out of wood or find 4 shells that were identical. Keep in almost everybody has a pack that has a some unique “defect” on it due to material avalibility and craftsmanship skill level. That said, I really like haveing a hallow fiberglass pack that I made from scratch so if you want to try molding more power too you.
As for materials for molding, you will need to get Rebound 25 brushable silicon from smooth on. It is a two part compound that cures into a strechable, non tear rubber with a distinctive orage color. To make a full mold you’ll need about 1 tub, which will run you about $191 after shipping. You will also need to get the thickening agent so you can build up layers of rubber.
The process I used went as follows: Prime your positive and allow the paint to dry fro 24 hours. In a plastic cup mix a small amount of equal parts A and B of the rubber. Be sure to mix only the quanity you need to cover the pack with one layer. Paint on the silicon and allow to sit overnight. On the next layer, mix a few drops (youll need a medicine dropper from the local drug store) to the rubber after you mix it to make it thicker. Keep adding rubber like this to the pack untill you cant see any of the primer, allowing each coat to cure overnight. Be sure to buid up rubber where the 90 degree corners are as this will be where your mold gets the most abbuse. When you are done with thick layers, add one more coat without thickner.
Before you peal the rubber off you will need to make a mother mold so the rubber holds its shape while casting. I made mine out of a layer of fiberglass cloth, then mat, then cloth to provide a smooth, unflexable shell. You will want to make the shell in segments. If you don’t, you will never be able to get the cast out of your mold.
To make the pack itself, you will need silica or talc for gel coat, bondo resin from home depot, and glass cloth. First you will mix equal parts silica and resin (follow the instructions for adding hardner on the can) and paint that in your mold. This keeps the outside from showing the texture of the fabric. You will then reinforce the shell with two layers of cloth and resin. Once the resin has cured, remove the mother mold, peel off the rubber, and you have a perfect copy of your original master positive. I have been using this technique for proton packs, traps, PKE meters, petals, and ODST armor with great results. Some picks of my molds can be found on my pack thread here:
There are a few items you will need to have a large supply of. Make sure you have the following before getting started:
Plasic disposable cups for mixing
Latex/Vinal glovers (fiberglass and rubber do not wash off easy)
plasic spoons
disposable paint brushes
And most importantly a resperatior for working with the resin.
Hope this helps.