I cut one out off 1/2" alum plate , drilled the mounting holes in it and milled fins . Never has leaked . Used the original 1/4" plate to make other things .
The plate was flat but being thin ally it had no strength with the two button screws so far from the edges, the orange seal was still good and proved okay.
The seep was very slight but it was enough to bug me into making something quick & simple to keep the original plate up tight against the shoulder, I had a bit of spare 1/4" flat bar so that fixed it, I'll probably do the same with a 1/2" ally plate in the future.
Hmmm.
I am still using the original plate that came with the bike 227,000 miles ago. I'm still using the original O Rings on the 2 bolts. The same rubber gasket has been reused for over 5 years.
I've never had a leak.