i would be looking at 3/4" flat rubber washers above and below the disk Plumbsure Rubber Washer, Pack of 4 | DIY at B&Q

22mm diameter is the same as copper tube

So a fitting such as Plumbsure Compression Straight Coupler (Dia)22mm (Dia)19.05mm | DIY at B&Q remove the olive and put in the washers and disk, tighten nut and put the body of fitting in a vice

you could use the 3/4" side of the above fitting with a brass cap Primaflow Brass Compression Blanking Cap - 22mm | Wickes.co.uk (hole drilled in top) with washers to get better mating surfaces

ER collets don't work with short lengths unless there is a balancing piece of almost identical diameter inserted in the back of the collet. 5C is a sensible choice for this. The problem is there is nothing to support the piece longditudanly and if you are applying pressure with the rod the piece is probbably twisting. That's why I suggested a pot chuck below (the thread structure of this Forum is not very good :-( ) This will also take the longditudinal pressure and it shouldn't be difficult to get one turned in aluminium that will fit your existing 5C collet.

The forum is a bit confusing. Occasionally, I get notifications about my own replies too rather than other users?

Anyway, let me clarify what do you mean. Are you saying create the fixture, split it and then use it in a normal 3 jaw chuck or fit it in a collet with the pneumatic fixture? 

Also, won't there be wear very quickly on the aluminium fixture?

Thanks for your reply by the way.

Yes, it will work in a 3 jaw or a collet chuck. Here is what I have in mind:

If nothing slips aluminium should work well enough to show if it works or not. The clamping pressure can be increased by extending the slit across the centre.

What are you doing, friction welding?

That makes sense. I might try that out next week.

Not friction welding. I'm developing an SOP for product assembly and this is one of the parts - it has to torque into another part and that's where the grip problem comes in.