Filed and sanded the letterbox plug to fit with a Shinto rasp and 40 grit disc on a DeWALT random orbital sander.
Marked high spots with pencil, sanded small sections at a time.
The plug fits, a little snug, so we'll sand just a tiny bit more to allow room for expansion and a little room for the adhesive. Thickened epoxy is formulated to fill small gaps with light clamp pressure, so a little space is desired, vs glues that need tight joints and lots of clamping pressure.
Log of ZIP.