I received the $11 drill bushing I ordered from McMaster-Carr (A 1/4" OD, #40 ID steel bushing), so with the limited time I had today, I drilled the main attach bolt holes in the elevator horns.
First, since the bushing wouldn't quite fit inside the horizontal stabilizer bearing, I chucked the bushing in the drill press and turned it down (with 100-grit sandpaper) until it would fit. I inserted the bushing in the bearing, and slid a 6" #40 drill bit through the bushing to check the position of the hole on the horn (remember that an AN4 bolt head has to fit alongside the bead of the horn weld), then I drilled the holes.
I drilled the left horn first, then removed the left elevator, installed the right one, and drilled the right horn. I'll enlarge these holes to slightly less than 1/4", then ream them for a perfect fit.
I noticed that the plans call for an AN4-15A bolt in this application, but after measuring a bit, I think I'll need at least an -18A.