Peter, made this for my current Roadster a couple of years ago
If I did one again, I wouldn't bother with the step-out, makes it more complex just to gain 1/2"
How did you get the correct measurement of the curve of the transmission tunnel?
Easy, measure width of your tunnel, halve it, set compass and draw the semi circle on some thick cardboard, test it (not forgetting a little clearance for the leather to wrap under & behind the plywood