OK, had a look at the bracket on my bike. It's a sod to get the shape, probably easiest to use a lock to get the right edge shape as the profile is the same.
There is a protruding lug on the underside which locates in one of the holes on top of the lock so not easy to trace the outline.
A selection of photos of the bracket, plus some possibly useful dimensions.
Diameter of lock plunger hole: 17mm, tapering out to 22mm on top.
Thickness of bracket: 4mm
Edge of shock absorber mount to centre of plunger hole: 113mm
Edge of frame tube to centre of plunger hole: 20mm
Top of bracket to bottom of frame tube: 4.5mm
Hope this gives enough information for a start at least. I'll try and help more if you need it but will be away for the next week.