STRONG STRUCTURAL FILLETING MATERIAL
While building TOAD HALL, I occasionally found that I had to fill a gap where I had not beveled a fit. I found what I think is the ideal material for this purpose; ordinary  household insulation fiberglass. I use the pink stuff because I found that it changed colour with the addition of the resin and I was able to discern exactly when it was fully saturated by this change in colour. I tear off whatever I need from the batt and stuff it into the gap, then pour in the epoxy. The fiberglass soaks it up quite  quickly. It starts out changing to a slightly darker pink then, as the epoxy soaks in and you add more, it will turn a very dark red. That's enough! It is fully saturated. The multitude of thin fibers in the fiberglass I feel, add significant structural strength to what would normally only be filler. One thing to be aware of though; the epoxy/fiberglass insulation mixture has considerable density and this means the the set-up time will be much faster than with just ordinary epoxy coatings. Be aware!

The finished fillet will be a little rough, (all those little fibres sticking up), but you can sand it smooth and add a little smooth filler on the top to even things up. The photograph shows the fillet between the cabin sole and the hull sides of TOAD HALL. I did NOT try to fillet this 1 1/2" of plywood sole but instead left the sides straight. Then I filled it with the structural fibreglass insulation and smoothed it off with the smooth filler.
Worked great!

LATE BREAKING NEWS!
A friend of mine who is mentioned elsewhere in these pages, G.B. Fisher, has contributed a refinement to the above. He gets fiberglass batting which is meant as pipe insulation. It's about 8" wide by 10' long, by about 1" thick. He dips it into a trough of epoxy, gets it thoroughly saturated, and then stuffs it into the cavity that he means to fillet. It's easier to use he feels than the larger fiberglass batts, and easier to manage in his dip and stuff technique!