a'n't

Fowler prescribed "a'n't" and not "amn't", for the reason of euphony you gave.

On aren't I DC 2009

He then bowed to him again, and begged pardon of all the ladies; but, in quitting the booth, contemptuously said to Mr. Dubster: 'As to you, you little dirty fellow, you a'n't worth my notice.'

Camilla 2008

However, I've got a boy to go and get me a pair; if all the shops a'n't shut up. '

Camilla 2008

But if ever I detect any of those monkies mocking us, and wearing our feathers, when you a'n't by, I sha'n't put up with it so mildly.

Camilla 2008

A little girl then, with incessant low courtesies, appeared, and looking smilingly in her face, said, 'Pray, ma'am, a'n't you the Lady that was so good to us?'

Camilla 2008

Only I've got no stairs yet to it; but there's a very good ladder, if the ladies a'n't afraid.

Camilla 2008

I thought they'd been quite young; I wonder they a'n't married! '

Camilla 2008

Then, turning to Lynmere, 'I wonder,' he cried, 'you a'n't ashamed of yourself!

Camilla 2008

Sure, a'n't I the man that patronizes your Melodies? '

The Atlantic Monthly, Volume 06, No. 37, November, 1860 Various

I've lived to bless the Lord that kept me from you, and I a'n't going to take my blessings back.

The Atlantic Monthly, Volume 06, No. 37, November, 1860 Various