This easy to make tool doesn't require any mods to your lathe, well mine at least, but the idea should apply to most lathes with a screw on cover at the gearbox end.
This is what I started with
And here are all the parts
I rummaged through my box of gears and sprockets to find something with 24 teeth, a number divisible by 2, 3, 4, 6, 8, 12 (and 24 of course). All regularly used workshop divisions. This sprocket came off a small obsolete and junked printing press I dismantled a few years ago, which also had lots of handy motors, shafts, gearboxes, bearings, springs etc etc.
The shaft is 40 mm in diameter which fits neatly inside the headstock bore, onto which I have machined 2 slots at right angles. The 10 mm hole through the centre has a Dynabolt (with a conical end) which, when tightened expands the slots to firmly grip the inside bore of the headstock. On the right is a spring loaded pin which engages in the sprocket to hold the headstock (and attached chuck) firmly in place to stop any rotation.
The business end of the shaft showing the slots and countersunk hole to engage the Dynabolt.
The sprocket lightly, but sufficiently welded onto end of the shaft.
Spring loaded locking pin made up from bits and pieces from the junked printing press.
Look closely, red dots to index four, yellow dots for 6, and blue dots for 3 divisions. I just use nail varnish, cheap and dries quickly, use it everywhere for things like this.
And finally with the addition of an aluminium disk I made up and bolted on later with 10 indents (machined on a rotary table) to accommodate indexing 5 and 10 divisions (eg for dial making).
I leave the spring loaded pin assembly in place all the time and just insert and tighten up the Dynabolt on the shaft when I want to mark out parts for machining. The nylon ring is now redundant with the addition of the aluminium 10 indent ring. The whole setup isn't really made to hold parts in the chuck with extreme rigidity, I use a rotary table for heavy duty work and for more complex dividing. But it takes a whole lot more time to set up, which is why I devised this quick and easy solution.
Hope this also helps you.