I have two tailstock turrets and find that drill chucks get in the way a lot of the time. One of mine has ER16 collet chucks instead and gives you a lot more room.

Not so easy to find in UK. Call Rotagrip and ask. Call Gloster Tooling and ask.

As above, 3/4″ straight shank ER chucks are plentiful from CTC Tools. Sell the excess Jacobs chucks you have and invest the proceeds in a modern tooling system.

Arc have 16mm ones in JT0, 1, 2 so you might be able to sleeve them.

Rotabroach items are always described as 3/4″ Weldon shank, so that could be an area to investigate but I do not know if they use JT chucks or threaded chucks as the norm.

Some available from US via Amazon:

These people have a selection, but are in US:

https://www.ebay.co.uk/itm/354300167202

Buy some 3 Morse to Jacobs adaptors (smallest size that will contain the 3/4″ shank – Chronos have 3, 6 and 33 JT very cheap. They also have JT3 on 1/2″ parallel, which could be sleeved) and remove the excess material. You can cut down a 3MT female socket so it leaves enough of the male taper of the JT adaptor projecting for you to turn it down (cheap CBN insert from China) to 3/4″ parallel. The cutting forces will be forcing the male into the female so the reduced length would not be a problem (light tailstock support will also assist).

The “batch of blank arbors” above is OK for multiple copies of the same thing. But if you have one chuck with each size taper on it, setting up for that taper might cost more time than buying an item with the taper already on it.

As Jason says a piece of pgms wouldn’t take long to machine, worthwhile if you have a decent batch of parts to make and also a great space saver (you won’t catch your hands so easily on all those sticky out bits!)

Hopper the collet chucks where homemade.