Leather fillets do work great. As do fillets made from balsa! (of course, the balsa takes more time and patience). The leather fillets are real easy to work with. Titebond, CA, Ambroids, Elmer's Glue All, Duco, etc., etc., will glue them on! Take you time where the fillets meet on the LE of the wing and stab, fitting the two end pieces together nicely isn't hard, but could be tricky the first time you use them. They look really good, are easy to apply, and accept all types of paint.
The leather fillets (IMHO) are great on OTS planes, and such! They really look good on them. I do use the blue Super Fil (I *think* that is the correct name, the jars are in the basement! LOL!!) since I feel it is a stronger fillet on my bigger planes. Maybe, maybe not!
But it is an epoxy like material and seems like it should be stronger to me, but I ain't no "injuneer", or scientific type!
Big Bear