Three polyester options from Turners Retreat come to mind which might suit your needs:
1) 'Alternative tortoiseshell', which is 20mm diam x 140mm long:
https://www2.theturnersworkshop.co.u...ternative.html
2) 'Alternative horn', which is 20mm diam x 130mm long:
https://www2.theturnersworkshop.co.u...ternative.html
3) 'Briar', 20mm diam x 130mm long:
https://www2.theturnersworkshop.co.uk/briar.html
Not expensive at £2.25 for the first two and £2.45 for the last.
Polyester turns well with light cuts and sharp turning tools, and will polish to a high gloss if desired, but like any plastic, will tend to chip at lower speeds or too heavy cuts. The main problem with any plastics is that the waste comes off in ribbons so you can soon be festooned in it.
I appreciate of course that the micromesh pads and polishing compound I mentioned in me earlier post are expensive for a one-off project, but given that you need 16 buttons in all, and that they'll be a prominent cosmetic feature of the radio, maybe not that expensive per button.
Every success with it.