RE: Building a cowling plug
To be very honest, I stopped using polyester resins of any kind some 20 + years ago, if I am planning on a painted balsa wood airplane then I simply use Stits covering system on all open bay areas and all sheeted areas get 3/4 oz. glass cloth, and Deft Sanding Sealer, or Minwax Polycrilic, after a couple of coats and light scuffing I prime, sand, and paint. There is just no need for the use of finishing resins IMO.
If I am going to build epoxy hobby parts then my resin of choice is E-Z Poxy 10 and E-Z 23 or 24 hardener, but that is just me.
Bob