I'll use accents. O is short, ó is long, ö is short, ő is long and so on.
How extremely neat!
I'll double vowels, man is short, maan is long.
But if the word goes on, single vowels are already long, e.g. manen, so we double the consonant after it to make it short, e.g. mannen.
I'll add an h after vowels to make them long, especially e, not so much after i. Or double them. But hand-written ii would look like ü, so I'll replace that with ie, and uu, ää, öö and üü are simply not allowed. And single vowels before single consonants are long anyway, so I'll double consonants after short vowels.
Your spelling is going to be a total mess, but at least reading will still be completely clear in almost all cases.
I'm going to add an e
… at the end of the word.
E.g. hat is short and hate is long. But after v I always add an e, so gave is long but have is short.
Chinese and Turkish:
We don't really have short and long vowels, everything is kinda the same length.
Probably a good choice.
Arabic and Hebrew:
We're just not going to write the short vowels.