My super-cheap yet effective solution:
I went to Home Depot and got one of those big, long connector nuts, the kind
you connect threaded rods with.  I brought my little calipers with me (or use
a ruler) and searched until I found one that was close to 17mm wide.  The one
I got was a tiny bit bigger, so I lightly ground down 3 sides of it until it
fit.  It was VERY snug, but I tapped it in, put a wrench on it, and it
worked.

Cost: a buck or so

One caveat: be aware that this is not the hardest metal, and if you don't
make it snug you can screw it up.
(Ya know, now that I think about it, the first time I took off the plug I
used a tranny bolt and vise-grips.  Even cheaper.)

Bill Marshall
85 GL Tiico "Pandora's Box"
Aurora, IL