I tried searching and didn't see anything, so forgive me if this has been posted before.
I recently ran across this product on a youth hockey board, and even though it's not a "goalie" product, I thought some of the coaches here might be interested.
"The First Detachable Letter System for Youth Sports Teams"
The letter (C or A) has snaps on the back of it, and the backing piece has the matching snaps on the front of it, so you just put the backing inside the jersey and snap the letter onto the backing through the jersey's material. You can assign a different Captain and/or Assistants for every game, and nobody has to arts-and-crafts together bits and pieces of stick tape onto the jerseys to make the letters.
And oddly enough, for a hockey product, it actually seems to be sensibly priced! For under $50 (including shipping), you get one C and two As in both home and away colors. (They have other groupings available, even just single-letter options.)
[Disclaimer: I don't know the people who run the company or anyone who works for them, and I don't benefit from anyone buying this product -- this isn't an advertisement, but simply a "hey, check out this product" type of post.]