override TupletNumber . Custom text

[Nils]

Hello, 

I like to work with full tuplet numbers because it makes it clear which version 
of quintuplet is meant (2:5 or 4:5).

Since I don't like errors, however 'traditional' they may have become, I would 
like to change the "3:2" of a triplet in a more correct "2/3".

How can I get this format into

\override TupletNumber #'text = [...]   %old: #tuplet-number::calc-fraction-text


greetings, 

Nils