The plenum I received from BillL fit my standard Van's baffling like a glove once I removed my black baffle material.
Unfortunately there seems to be some difference in the vertical position of the front of the plenum. My baffling for the Van's cowling is trimmed to a tight 1/2 inch all the way around, that leaves about 3/4 inch of metal baffling above the front corner of the #1 cylinder. Seems the Sam James cowling must have more vertical clearance in the area, Bill was nice enough to send me a template of his front baffle for the #1 cylinder which is approximately 3/4 inch taller than mine. In order to keep 1/2 inch clearance from the top cowling I had to lower the front of the plenum.
The piece of paper shows the bottom of the top cowling with a 1/2 inch clearance line, the plenum is well below that.
I was hoping to possibly trim the outboard front corners slightly, but I think there is just too much variation in the shapes between the Van's and Sam James cowling.
Although this doesn't look like it's going to work for my installation, I think BillL's Plenum is a great product for anyone installing a Sam Jame's cowling. His product, price and service was unbeatable.