Some years ago I was in a Home Depot garden section and I happened upon some rolled Pond Liner. This stuff looks identical to inner tube material but it comes in 12-ft widths (you buy as many feet as you need).
This stuff is perfect for, and it is what I used on my electric fan, mounted on the FRONT of the rad. (Yeah Ray, it's the one you sent me from your bone yard.) The electric fan has a plastic shroud so I screwed rubber strips to the inside lip, all the way around using short trim screws. It works nicely with no leaks.
You guys with shrouds behind the rad can screw it to the outside of the shroud as air draws into the fan.
I cut my strips wide enough so that the rubber edge cannot vibrate on the cores. Instead, the rubber sits flat on the rad.
These Ford fans are two speed and reversible. I always use relays on my electric fan motors and I fuse them. To make them reverse, simply power the ground wire with +12, and use relay contacts between the Hi and Lo wires and ground.
Why reverse? Because I didn't have enough room between the rad and water pump. So, I pulled the rad back towards the pump ~1-1/2" with spacers and mounted the fan in front. Now the fan motor pushes cool air which is much more dense and the electric motor never gets warm. This was a good modification. - Dave