so the tube is 14mm ID and you take the 1/2 at 12.7 mm and divide that in half and you have nothing left at 25 thousands
now that picture show a piece of what looks like delrin collar
so I would get a delrin washer made to fit the aluminum adapter and shim the adapter internally with some flexible and take paper punch and punch holes in it
and epoxy it al together and my go to epoxy is J-B Weld 8265-s It is 24 epoxy and will clean up easily with alcohol and is very strong
now the hard part where to find that washer off the shelf, well after look on Ebay for delrin washers it seems that Polaris uses them but no sizes give
https://www.ebay.com/itm/Polaris-ATV-Replacement-Shock-Bushings-Delrin-7041454-Made-in-USA-with-washers/282894745380?hash=item41ddd73f24:g:qh0AAOSwBPFassj2 you could get one made and I think have someone 3-D print that tube end finisher would be my choice at this point
now you put a haha after say you would like it to look like a pro did it, remove the haha because you are trying to do something with limited tools and maybe know how and the fact you are trying and learning as you go is a very good
you know the difference between very good and bad and knowing that you are far ahead of the game, I'm 69 years old and many projects I wanted to do but never did was because I knew I could not achieve the level of quality that I wanted with the tools I had, have
I still struggle with that fact today but with a very good imagination on how to do it and the guts to try, thing can be done but failure is certain
so have some print that end piece or have someone turn a washer
about all I have I can be reach thru messaging
take care and good luck mike
material ideas
https://www.ebay.com/itm/2-pc-Extsw-9-mm-ID-x-20-mm-OD-x-3-mm-Thick-316-Stainless-Washer/113824701037 https://www.ebay.com/itm/50-100Pcs-M2-M2-5-M3-M4-M5-M6-M8-M10-M12-M14-Plastic-Nylon-Flat-Spacer-Washer/153061831436?var=452894468396 https://www.ebay.com/itm/Nylon-Flat-Washer-Plastic-Repair-Penny-Mudguard-Washers-Gasket-Black-M2-M20/153336481860?var=453234680453 https://www.ebay.com/itm/100pcs-Black-Plastic-Nylon-Flat-Spacer-Washer-Insulation-Gasket-For-Screw-Bolt/153245660857?var=453112720224 all would have to be reworked for ID the stainless could be done by the seller but could be added weight all Ebay seller from China I have had great service from 14 to 24 days to the west coast and are worth a try
the first try might not work
I might have tried one of this over some sort of 14mm insert for the shroud ID like carbon fiber they are steel and could be more weight then needed but they are blued so the rear could be cut down and reshaped to match your 20mm OD profile
but that's just my take on things
about all I can do
I hope some of this Is helpful