There are members here -
flh801978
for one - who do this type of work. It might be worth putting something in a wanted ad. If you find someone it might be worth asking which route to take - an insert or a backplate - before purchasing either.
Yes, I was thinking 3/4". If you deemed it worthwhile someone could open up a 3/4" x 16 for you. You might find it hard to find a 7/8" insert for the others.
If the finances can stand it, kits similar to this are very useful.
https://www.axminstertools.com/axminster-2-mt-multi-head-live-revolving-centre-additional-tips-718222?queryID=fd190da3b62f7a0085f488fc371da918
Actually the landlord of my old local used to sell mixers in screw top bottles - 250ml, iirc - to locals who put their name on them if unfinished, it was cheaper pro rata for him and them to buy. He reckoned he sold more shorts that way.
I have a large bottle of TruOil bought before my life changed course. I read here somewhere to put a pinhole in the foil and just squeeze a few drops out as needed - it seems to work, it's 13 or 14 years old and still OK.
Ah ..... unusual for a modern machine. You could get a Versachuck and get a plain backplate threaded for the left side. With an ordinary backplate you could use the same chuck either side...
I used to use marbles to take up the air space when making wine, not wanting to dilute the flavouring. I have a bottle of cellulose sealer with ball bearings in it - it's easier take shake up.