They were made using either 'Leelawadee' or 'Century Gothic' fonts, depending on the character (with a capital 'I' rather than a '1').
Cell size was 3 wide, 15 high.
They are font size 13, with a space between the 2 characters that varies to ensure the gap is correct (eg: the space between the double 0's is set to font size 5, and the gap between the 1 & 0 is set to 11).
They are printed on glossy photo card, 260gsm, using a canon inkjet printer.
I removed the headcode glazing, scratched the front a little to key the plastic, then stuck the headcode on with superglue. Once dry, I trimmed the edges using a sharp scalpel, then ran a black felt tip over the white card edges.