Earth Auger shaft adapter 7/8" to 3/4" DIY

I had a very hard time digging the hole with a demolition hammer a couple months ago for
installing a dog house sail.
It was so heavy for my age.
So, Purchased used post hole digger from the local craigslist for $50 for future need.
That is a powermate 43cc earth auger with 7/8" shaft.
But it has only an 8" auger which is too heavy and I don't need that big.
Purchased a 4" auger from the Ebay for $26 which is only 4lb and great feedback but the shaft is 3/4"
I have to make an adapter for the shaft from 7/8" x 3/4" .
Now, it is lightweight and easy to handle.
John's Ranch