Positioned fin on stab and marked outline of fin bottom on stab. Marked and drilled pilot holes on narrow support bottom. Positioned and drilled pilot holes. Ended up shortening the supports a bit because of the stab surface curve. Upsized holes to #30, prepped parts, and riveted.
Replaced vertical fin and marked rivet line on fin skin and drilled the line. Deburred parts and cleco'd only at this point.