Ivan located a scrap piece of 3/4 in aluminum the right size for the adapter plate. Price: free!
---
And as if I don't have enough projects, I've been itching to make a trailer for my beater 10-speed (which I spend more time on than in my car in the summer). Half the time I fire up the car these days, it's to bring something down to the boat that's too big to carry in my knapsack on the beater bike. Which tells me: time to get a bike trailer.
I have all the junk needed to make something like this ... including the electric motor... so it could also be a "pusher trailer".
Details:
http://www.electric-bikes.com/bidwell.htm