Epoxy
Many years ago I built some stocks using polyester, not good.
Many reasons to use epoxy over polyester, stronger, doesn't stink, better bonds, able to get insurance on a commercial level, doesn't stink,etc.
To make a production mould we used tooling gelcoat and lam resin from the same company for making tools. The gelcoat was filled with silicon carbide to increase cut resistance and this stuff was much more stable and could go in and out of a hot box 1000's of time and not distort and was cheaper to buy. For a quick one or two off mould we would use the regular epoxy that we made stocks from.
Never used mat or a chopper gun to build stocks. Although mat may seem stiff and strong it is not as strong as woven cloth by a long shot. Also important is that whatever fabric you use must be sized with a chemical compatible with epoxy. The sizing helps it stay together until wet out. Remember that all the strength comes from the fabric not the resin (more or less).
The main cloth was a 7500 weave tooling cloth in six and ten oz weights. We also used six oz cf bid and uni. Bid = bidirectional and uni = unidirectional. We also used a number of bulking fabrics for making moulds, the names of which escape me right now. This is all fairly high tech stuff you can't buy at a boat shop. For small quantities Aircraft Spruce and Speciality is a good source and they also sell small bits of 4140 sorts of steel if you need it for anything. We used an aircraft approved epoxy and although expensive the cost per unit was not too bad buying it by the drum.
Measuring epoxy is best done on a scale although small quantities can be dispensed with an epoxy pump which is very accurate for small amounts. If you don't want to spend money on a pump buy a small electronic scale for <$20. I always work in grams because it is simpler to work in whole numbers all the time instead of fractions, not an imperial vs metric thing, just easier. If the seller of the epoxy you use can't tell you the ratio by weight then buy from someone else. Figure out what the ratio is as 100:x. For example Marine Tex says the by weight ratio is 6.3:1. Hard to work with but if you turn it into a percentage ratio it's easy. 6.3:1 = 100:15.873 or 100:16 should be close enough. Put any old container on the scale, zero it, put in the resin you will need then add 16% to the weight using a calculator and add hardener to this number. The lam epoxy I use is 100:38 so to get about an ounce (for those die hards) I would add 25g of resin and then add hardener at 38% up to 34.5g. Perfect every time. Mix the living crap out of it and if mixing something thick like Marine Tex: mix well, dump it into a clean cup, mix well again with a clean stick and then use another clean stick to apply it. You only need one screwed up job to make all this mixing worthwhile.