Unfortunately you can’t mix copper and aluminum in a water loop, get a reaction that costs the inside of the radiator and waterblock(head in this case).
My other concern is the “plastic” piece had groove protruding in the shape of the radiators loop. I’m wondering if that separates the channel from just running together through the open space. I’m not sure how well it will seal that area without the plastic.
Possibly some thing aluminum sheet could be made into a gasket?
Definitely correct thinking that the backplate offers limited cooling benefit. Nonetheless a benefit if I can make it more efficient.
The other major thing I noticed is this radiator is really only cooled by blowing through those open slots in the middle. If this radiator is mounted parallel to the frame, or right up against the underside of the tank, it won’t cool efficiently.
It’s seems between the forks or something similar is key, doing that with an efficient thermosiphon will be a PITA.