Capture a Reference Code

Hi. I’m trying to capture a code - e.g. ABC123F4G5
I’d like the user to say one character at a time, which Alexa repeats back (with the option for the user to say “wrong” or “start again”) at any point.
At the end Alexa spells back the whole thing, gets confirmation fron the user, then concatentates all the characters into a variable.
(As an aside, I concatenate at the end rather than after each character capture so that the user can correct eg the 6th character - all without getting into code. If I didn’t need that then a loop with a boundary variable to manage the number of loop iterations would be ideal).
Anyway my issue is … I can get it to work in the VF test interface, but when I test it in Alexa it doesn’t recognise any of the characters.
I suspect that I need to use phonetic synonyms - eg “P” / “pee” but before I do I wanted to see if anyone had solved this already in a neat way …
If not I’ll be happy to collaborate to do so and then share it in the marketplace …
Please let me know !