I mostly use a more or less aero-space approved laminating resin
from a company R&G in germany. They have one of the greatest selections of resins and reinforcement materials. Their website is also in english and has a shop.
What's important is to have a nice long pot life with the epoxy mix, otherwise it just becomes a rush job. I use between 40 and 90 min type.
Filling the webs in a fore end of a plastic stock can be done by mixing ultra light micro balloons with resin and the covering with woven glas or
For bedding one can use ground carbon fibre, short glass fibre or aluminium powder. Nice thing is one can mix all or whatever one wants.
The viscosity can be adjusted to the job.