Earlier machines had smaller numbers of a different style and the layout did alter over time. If you give your census number, perhaps we can bracket it with actual examples from period photos to give you a better idea ? There are no modern fonts which are exact copies of the ex-factory forms.
What technique will you be using for applying them ?
Bob's bike is from contract C9387 and I calculate his census number to be C1034476 from about mid 1941. Ariel didn't seem to alter the size and font much according to these two pictures from O&M.
I used 1" scaling it against the tank badge and style 1 here..........
Ah, an Ariel. I tend to assume that us non-Beeza chaps will usually mention the make. C9387 seems to have been one of the last motorcycle contracts to use those RASC serials.
They certainly do seem to have followed a practice of small figures set high and not conforming with the standard regulations. It would look odd with the large BSA style numbering.
I thoroughly recommend something in consultation with AxHolme signs as well. They'll send you proofs and help you to arrive at just the style that you're looking for.
Unfortunately, whilst I seem to have quite a lot of saved Ariel photographs, the quality is generally pretty poor. No decent high-res factory shots.
If you copy this TrueTypeFont on your computer (in the C:/Windows/fonts directory on a MicroSoft computer), you can make your own stencils in Word, by using the LaserPaket font.