The one Jason suggested is very nice.
If you search 'boring bar micrometer' and look at images, some of the results will give you some ideas. Any that include 'Van Norman' are not what you are looking for – it is the ones that look like a standard micrometer with the fixed anvil replaced with a vee block.
A vee-block with a micrometer barrel strapped perpendicular to the vertices of the two vees would work. Replace vee block with an old centre-square body and add micrometer.
Grind a notch in the cutting bit diametrically opposite the securing screw. Use a second screw with a tapered end engaging in the notch to advance the bit. Very limited range of movement with this though.
Use a normal micrometer across bit and bar after doing an initial cut and measure. If diameter of cutting bit is so large that micrometer anvil falls into the hole, use a gauge block to span the hole. This does mean you have to shorten the butt end of the cutting bit shown in the photo.
See also:
https://www.model-engineer.co.uk/forums/postings.asp?th=130528
https://bbs.homeshopmachinist.net/forum/general/37455-boring-bar#post721214
OK, found it. Search for Moore & Wright 462.
Edited By DC31k on 31/12/2020 10:38:52