Like the other current Meddings thread, I would direct you to Vintage Machinery’s web site where there is a wealth of manuals and other material for instant download That link takes you straight to the exploded parts diagram of a Mk2 Driltru, from which I took this magnified screenshot of the belt cover retaining thumbscrew:

Combine this with the dimensions provided by Noel and you should be able to come up with a suitable replacement, even if it’s not an exact replica. The picture on the lathes.co.uk that I looked at wasn’t quite as clear as the one above.
John