i am not sure whether you are trying to make a composite or just the transparent resin casting alone. The former is pretty tricky as it requires precise refractive index matching with fibre (glass in most cases). Transmission would depend on the type of fibre, tex, weave pattern, resin system, cure schedule, curing agent, etc. Acrylics are a good starting point with ICI having some good materials towards this end.
good luck
/SS/