Actually, the most commonly promoted spelling is 'misspelt'. While I'm generally in favour of keeping the strong ending, here I don't like the resemblance to the grain spelt.
I mainly hyphenate because of the successive 's's, although I also like the little joke in that.
As you've pointed out:
'spelt' is a grain
successive consonants, which are not hyphenated
Keep making the effort, and you'll get there in the end - 6.31/10
As you've pointed out:
Keep making the effort, and you'll get there in the end - 6.31/10