So I ended up using the 3/16" thick vinyl tiles from HD (they do have large dimples on one side, but don't think that will matter). I initially cut several of them out to match the aluminum base plate (first pic)...which was pretty simple to do w/ a razor knife...tracing around the plate.
Aside from elevating my foot another 3/8", two goals were to minimize weight, and not compromise the plates ability to flex...so I elected to stack two of them on top of the plate, and trim them shorter -to be not much larger than I need. Also drilled (6) holes for the toe bar (second pic).
I covered the top layer with 3M grip tape. Final product is in last picture. (Yes, I realize I don't have a big toenail...but I couldn't afford a foot model, and don't know how to photo shop).
Obviously I needed (6) longer machine screws for the toe bar. Turns out the #8-32 x1-1/4" ones I picked up were a bit too long (and 1" is not long enough)...so I just ordered a pkg of 1-1/8" length on ebay...which I'm now waiting for....for final assembly and test.
Hopefully this info is helpful to someone... or at least interesting.
One downside of this was that I had to purchase a 6 pack of the 18"x18" tiles ....so I have extras. Three "spacers" can be cut out of each tile. If anyone wants some, drop me a PM.