does anybody know if there is a design for an index pulse board
You don't need anything special.
Hood and myself have used the same and quoting Hood " an Optek OPB917B it has everything you need except for a limiting resistor for the LED, 220 Ohm is what I use when powering from 5V". Use just the opto on the Sherline lathe.
Some folks put and additional circuit after the opto to make sure the signal is a nice clean square wave.
I currently use a Halls effect sensor as it uses a small magnet for detection. It is not to cheap to buy, works great. and made for easy mounting on my Atlas lathe.
Just note that some types of sensors will not be work well ie; a reed switch may not sense quick enough for higher rpm readout. It depends on the device. So read the device specs.
RICH