Bravo on the engineering solution.
Originally Posted by Thasiet
One thing you could try would be to drill and tap the fork - or even add water-bottle bosses as more solid mounts. These can be simple threaded "Riv-nuts" which many companies [e.g. Cannondale] often use to very cheaply and easily fit their frames with bottle cage or rack bosses. They just press in, basically like pop rivits. The tools are expensive, but the technology is simple, and many bike shops have the tools (and the threaded rivits) for effecting repairs or adding more bosses to bikes.
Then you could use simple 1/4" or 3/8" aluminum tubing cut to length, as spacers to center the rack. (you can usually find the tubing at a local hardware store in 1 to 3 footh lengths for only a buck or two. And then use long-ish 5mm stainless bolts. to anchor the rack. This "might" offer a more rigid mount... but your setup makes sense to me too.
Yeah, I'm forever rigging bikes to hold things (like alloy fenders) solidly in place. And older bikes, especially non-touring bikes fron the 70s, never had enough of even the most simple, practical, useful fittings - anywhere on the bike.
FYI... These plastic spacers are also easy to find at the big Home Centers.
Usually available in just 1/4" 1/2" or 3/4" lengths.
These particular ones are a perfect fit for 5 mm bolts.