I thought it was worth the time / effort to build a CFW myself considering the cost of the SBig CFW8 here in the UK.

The Mechanics'
I have used a delrin gear to mount the filters so that separate filter indicator pegs and IR detector are not required as there is a positive drive between the motor and the filter wheel. The micro just keeps track of the number of steps between filters and can return exactly to the same position.
There is a datum pip at the edge of the gear that allows the microcontroller to find the home position at power up.
The interface to the ST7 is the standard Sbig pulse width signal. I'm working on a RS 232 serial interface too.

The Electronics'
An Atmel AVR microcontroller and a bi-polar stepper motor driver ic and a few passive components on a prototype vero-board.
I'm working on a PCB which will be about half the size. The datum micro switch is on the inside of the housing at 2'o'clock.
A 6 pin header on the board allows the AVR to be in-circuit programmed, making program changes easy.

The CFW / ST7E assembled.
The filter on the 2" nosepiece is a IR blocking filter.

 
My M1 image was the first taken with the filter wheel.