The adhesive of monokote likes to stick to well sanded balsa and it will stick to glass just fine if the surface wax is cleaned off after the resin cures. If the fillets have spackle on them, the spackle will need to be sealed. Bondo works better for adhesion.
I use as low a heat as possible and it would be nice to have a special shaped iron, but I just struggle with the standard $20 special.
Shingling in strips of covering lengthwise with the fillet is the safe bet. If you encounter lifting, it's time to try again, or slit the spot that lifted and splice in a patch. The monokote video makes it look easy, but it's the hardest part of the plane to do.