Today I have cut the inner oil door from a piece of epoxy stock that was delivered with the SJ cowl. I clearly is made out of the same cowl template as the curvature fits perfectly with the one on the cowl.
I took the time to slide it in place on the top cowl and then gently pushed it very small amount to find the perfect spot. It 's important to check from the sides that there are no gaps between cowl and door piece. Once it's cut, there is no way back and the way it lays now will also be how it fits inside the cut hole. I tried it with the cowl laying on the ground and then again with the cowl on the plane to get the real curvature when fastened with the skybolt studs. Marked the location and reproduced the same location on the ground. I also used a thick sharpie to trace the outline of the oil door from the inside. You can see it shining through the transparency of the stock piece in the image below.

Then I used the epoxy cutting disc in a die grinder to cut the side close to the line. Finished the straight lines with a sanding block to keep the lines nice and straight.

When the lines were good, I rounded the corners.
Get close to the shape of the hole but don't overstress it yet. You will need to sand it  back more later on for paint clearance (0.050 gap required)

It fits perfectly now. Very pleased with the outcome. Now it's time to layup glass underneath tho support the oil door.