My boat isn't a seacraft, I have a 73 Chrysler Center console 20' . ended up joining here simply because of the level of expertise evident in the repair projects.
I needed 3 x 5 gal buckets of seacast but bought 2 instead to save $220, made up another 5 gals myself after checking the seacast out.
They package a lot of extra chopped fiber and I still have a load of it leftover so no need to buy more.
This book by John Wills has lots of resin formulas to make up your own concoctions.
He was involved in discovering how to make polyester resin catalyse at room temps 50 yrs ago.
For $16 its an interesting read even if you don't plan on making up your own mixes.
http://johnawills.com/indpu.html