Andrew,
Thanks for your interest.
I used MAS epoxy, Biaxial cloth, milled fiberglass fibers, and West System high density 404 filler.

After grinding and establishing a good base:
Coat area with unthickened epoxy
Fill low spots, and make the roundover contour uniform with thickened epoxy and milled glass fibers
Sand smooth and apply three layers of Biaxial cloth
Sand rough / high spots
Apply thickened epoxy with milled glass fibers
Continue numerous applications
Once you get close / back to the area where you'll want the crisp sharp edge, start applying your mixture with a little more grace
Trick here: Go beyound where you want the edge to be and then grind back down to the hull.
Trick: for the corners, use a 1 inch wooden dowel rod and 60 grit paper. This will make a perfectly shaped /contoured corner. Sure you'll be using hand power, and it will take a bit of time, but it really comes out nice.
When I'm grinding, I use a 40 grit flapper disc wheel to perform the grunt of the work. Then transition to a 5 inch random orbital sander with 60 grit, followed by 80 grit and then 100 grit. Final sanding will be done by hand with a block of wood, my dowel (described above) and 120 grit. Lastly two more coats of unthickened epoxy, light sand and ready for paint.

I had initially thought of building a form, but decided for numerous reasons against it. Please keep in mind that it took me numerous applications to rebuild the corner edge that I had removed when I initially ground away all the cracked gel coat. I really could not believe how much gel coat was there. I kept asking myself when will I reach glass? No kidding!

Hope this helps.