Yes, we'd ideally need to find a digraph that absolutely doesn't occur in Spanish. This can be tricky with compound words and loanwords. It seems that ou, oe and oo are super-rare in Spanish morphemes but can occur in loanwords and compounds. I just searched for unaccented digraphs that literally don't occur at all in /usr/share/dict/spanish and the only examples (of which there are 184 excluding k and w) contain only consonants and y.
So, there's not any easy natural way to do this without creating at least some ambiguities.
So, there's not any easy natural way to do this without creating at least some ambiguities.