Thanks a lot for the many ideas. Got the job done amidst some hilarity to do with what I tried first
I measured the original Michell's spindle dia which was 7.29mm and turned up an exact replica from some stainless roundbar and made it +-40mm long.
Then today's heat got to me (40+degrees C) and I was too lazy to get a ladder and bring down a piece of mdf from the rafters. So I looked around and found a nice piece of 15x15" aluminium plate.
I drilled a 7mm hole in the centre and press-fitted the replica spindle in the hole. And here was where the fun started.
The Michell's glass platter is bevelled so I reckoned on using an old knackered 12" shellac record as a cutting template but this was oversized as I wanted the mat to extend without overlap up to the platter's bevel.
I decided to turn the shellac down but it was too big for my lathe and mill. So I rigged a clamping arrangement for the record and to fit into a rechargeable drill chuck.
So here I stood with a spinning shellac on the drill in one hand and an electric file with a coarse abrasive belt in the other hand, grinding away.
I got about halfway when the disc shattered and pieces of Cortot on 78 flying everywhere
Undeterred, I made an adaptor to slip over the replica spindle after putting the punched cork sheet over it and put the glass platter upside down over the spindle.
Cutting the cork with the Michell's platter as a template and using a very sharp Stanley blade was a piece of cake. Snug, flat fit over the spindle and no runout.
At the time of writing this, the temperature is still in the high 20's at almost 23:00hrs and time to re-assemble all.
Never heard what a cork mat sounds like. Just imagine if there was no vinyl left anywhere. How can the CD spinners have so much fun?