So I used some 3/4" plywood made for concrete formwork. It seems of reasonable quality, has seven plies, and a smooth paper on one side. (Dumpster diving material from a nearby bridge project.) Used it on both sides of the aluminum, then had to add more thickness on the outside because of the lip on the transom. So I ended up with about 2 1/16" total thickness. This seems OK for all the OMC motors I checked, and even a Clinton. But it’s not OK for my Elgin/West Bends, limited to about 2". Will need to plane some thickness off the inboard pad, or make a new one. I could have planned this job better.