The "boss" or "flanged bushing" that you require depends on the shaft diameter of your motor. Many of the fractional horsepower motors that I've come across in the last 35 years have had a shaft of approximately the old 1/4 inch diameter. These form a snug fit with either:
(1) Radio control knobs - these generally include two grub screws for fastening.
(2) Flanged bushing from an old variable capacitor from a radio transmitter or receiver.
You can also find some flat gears that have similar grub screw fastenings. Generally, the relatively slow rotational speed you're contemplating - less that 800 rpm - will not demand any sort of dynamic balancing of the bushing, though careful centering of the scanning disc is desirable.
No hard and fast answers here, but you should be able to adapt something fairly readily.
If all that fails, you could have a suitable flanged bush run up at moderate cost by someone with metal lathe shop skills. Starting with a lump of brass would be the easiest and cleanest-cutting metal to use.
Hope this helps,
Chris Long VK3AML.