I can't see exactly where you are working but usually something like that would be done on the inside. Sand the surface where it is low on the inside. Put some tape on the outside as a "mould" to hold the resin from going all over the place. Apply some resin, put small pieces of mat there pushing it into the resin you brushed on with the brush and apply a little more resin and put another piece doing the same to push the air out of the matt or cloth what ever you are using. Let it go up much higher than you need and after it cures you remove the tape and sand the edge down to where you need it.