Haven't actually tested this yet, but here's what I came up with. It's cheap, easy, and looks good, anyway.
My plug seems to sit in the hole in one of two "modes." When I first insert it, one end sticks up a bit and the other end sits down on the end tab. In this "mode" the bottom of the plug isn't parallel with the bottom of the boat. If you shove down on the high end, the plug snaps into the other "mode," in which the bottom of the plug hovers ~1/4 inch (~6mm) above the keel line.
So it occurred to me that a ~1/4-in.-thick layer of some kind of foam glued to the bottom of the plug, and (unlike the plug) actually shaped like the hole, would do a good job of fairing. The only problem being I didn't know how to get my hands on 1/4-in.-thick foam. Then somebody suggested "craft foam," which I had never heard of. Sure enough, it's found in craft stores. I was skeptical but it turns out to be good stuff, and dirt cheap too. It comes in 2mm, 3mm. and 6mm thicknesses.
So, long story short, I got my hands on some 6mm craft foam, traced the hole, rough cut with scissors, and used combination of belt sander and drum sander to finish shaping (something I learned at work - most foams sand fairly well). A couple of pointers here: give up on the idea of a symmetrical plug if you want a tight fit - the hole won't be symmetrical. Also - if you trace the hole you'll probably have to take an extra 1/16th inch off all the way around, due to the chamfered hole edge.
I then coated the plug with contact cement, allowed it to dry, put the plug in the drive hole and carefully pressed the foam fairing on from underneath the boat, to assure alignment.
Here are some pics. It's sits almost perfectly flush over the last 2/3's of the plug's length. It''s ~1/8-inch high at the very front.
However I guess it proves how good the product is.
