Will writes:

How should I go around making a mask that I can blast thru to get the
required pattern of letters?

Thin rubber sheet (Enco etc) plus contact cement, if you want to cut by
hand with a razor knife. Inner tube repair kit rubber sheet will also do
if you just want to experiment with a sample.

For light cutting, computer-cut vinyl from a sign shop will do. A good
shop will also have sandblast mask material that costs more.

Reusable is tricky. Stencil shape and adhesive are not conducive. I would
go with a light blast and one throw-away vinyl stencil per piece.