Many thanks to all; very helpful. 

I am using 1mm Plasticard with a paper printed counter-set stuck on, which snaps nicely after two passes of the craft knife.  I fear it will take a good 10 hours ... at least.  Well my 2mm project took 50 hours of painting time so this should not be too bad

Total cost £47.65 plus hours of time plus saving a sanity roll.

Doing about 10 a day, spending about 5 minutes on it.  Thus done first 300 and taking a pause, just don't be in a hurry!

I remember the old school approach involved a hole punch (available in different sizes from office or leather work suppliers).

Material was frequently artist's mounting board, through a recall one grognard insisting on linoleum.