Hi
If you go on the official club website Douglasmotorcycles.co.uk you will find the model range, scroll dow to the approx year and you will find a picture of a Mk 3 Sports, note they are more squarer, they are not the kidney shaped ones as per the Mk One.
I have a Mk 3 but my Dad made mine from fibreglass - many months work. Mine are oversize so as he used some borrowed ones as a template. They are do not fit onto the bike in the correct way due to being oversized. Don't make that mistake.
There may be some members down your way who may have a Mark 3 Sports to visit & get the correct diminsions.
I think the ISDT machine has them also, so Eddy if your keeping watch, do you have any dimensions....?