I've used leather fillets since 1950 ! (I located a tool supplier in downtown Los Angeles who had them for patternmakers).
I use Ambroid. Wetting the leather works best. A piece of wet paper towel will do. I first tack down one end. When that holds, the rest can be wet and glued down. Ambroid grabs pretty quick, so the fillet stays down OK. While still sort of wet, I press the leather into a nice concave by using a round tool (I use a ball bearing epoxied to a length of brass tubing). Once dry, it stays in that shape.
Leather fillets tend to grab butyrate dope just fine, so there is never "pulling up" of paint from leather.
Good leather fillets are cut down to a fine edge. Even so, a slight "ridge" remains. Any water-based model filler can feather the edge. (epoxy fillers are too hard to sand, and you will mess up the leather by sanding it.)
Floyd