Speaking of an oil cooler plenum...
Noteworthy in this picture is the mount "ear" on the rear bolt on the cylinder. This is made of steel and is one of the primary load supports for the whole cooler. The remainder of the baffle (outboard portion) is very light aluminum, as it only has to contain the air and support the rubber seal.
I made a plug, then a mold, then cranked out the part in 8 plies of 6oz BID glass. You will note that the air opening is only above the cylinder, retaining the baffle. This not only reduces radiant and convective heat transfer to the cooler, it also keeps the air going past the fins on the head. I think this works out better for the cylinder, as this is my coolest one of the 4. The oil temps are also no problem, but the results have been matched by other's installations.
The picture of the brace shows the replacement of the way too heavy, emergency repair at OSH this year following the failure of an aluminum strap. The new (red) one is a weldment of .250 dia. 4130 tube. Much lighter, much cleaner.