I bought a bowl from Donkee, Its the bowl they use on their spinner.http://www.donkee.com/work_station.htm
It has a molded circular indent on the bottom of the bowl which I took a piece of 1/2" plastic and turned it down to fit the hole which perfectly centered the bowl on the shaft.
You cannot add what you call a dimmer to a 120 volt AC motor, it will not work and you will either kill yourself wiring it or start the motor on fire.
The only way you adjust the speed of a motor and it has to be a 3 phase motor, is with a frequency drive.(can be done with a DC motor, but that's a bit more complicated)
I did build mine with a 5 to 1 ratio constant torque 3 phase motor, meaning a 1800 rpm motor can be slowed down to 360 rpm.
I then bought a 240 volt single phase input, 3 phase output frequency drive to control the motor, I have infinite speed adjustment.
I'm a little weird this way, I wanted to prove i could do it, would have cost about 300 bucks with this set-up.
You are way better off buying one, get the 2 speed one if you do.
I have heard of some folks putting a bowl on a adjustable speed drill press, but the lowest spindle speed I found was 650 rpm which is a bit fast for sanding.
Drill presses are cheap on craigslist.
450 rpm for sanding, 650 for polishing