I have seen many self made boats using the stitch and glue method, both from kits and plans. And I have seen fillet material size ranging from small, 3/8 inch radius to very large 2 to 2 inch radius where the builder thought that if some epoxy is good, more must be better.
My experience has been that only the amount of fillet material that will allow an overlaying fiberglass reinforcement to lay smooth and remain attached to the substrate wood is required. Anything more just adds unnecessary weight.
I prefer epoxy and cotton fibers as the filler and keeping the fillet small. The cotton adds some structural strength but lays well and is color neutral.
Others prefer wood dust or mico balloons of glass or phenolic, cabosil, etc.
For the experts at CLC: How much fillet is necessary? What is the strongest fillet material? And what are the weight/strength benefits?