I was going to make a template to do this but found an ideal one in my drafting equipment. It is an acrylic protractor. It is the exact size I wanted.
First of all I marked out some base lines 40mm (just over 1.1/2") up from the top of the upper mortise. Then I marked out another line 60mm (just under 2.3/8") further on from this line. Then I simply joined the dots. I projected this around to the other face and marked that too. This is just as a visual reminder when cutting. You don't want to cut the wrong piece out at this stage!
|A protractor used to mark out the curve|
|Two mirror images|
|The curves finished.|