Interesting, and no. I would use the ones like EbayEd has.

The problem with the ones you suggest is that the motor mounts from the motor come directly over the stringers, so I can't just bolt the aluminum angle piece to the side of the stringer, because then the main stud of the merc style motor mount will not line up. I guess I could mount the angled piece of aluminum so the horzontal leg is over the stringer, but then I'd have to raise it to get bolts in to attach the merc style mount to the top. Which then might make it too high for the motor.

I fabbed my own several years ago. I welded the adjustable mount to a metal hanger that went around the stinger. It was a like a 6 inch box that I ran bolts thru to hold it then I welded the mount to that.

the ones in my picture are on a piece of angled aluminum and the mounts would sit directly above the stringer