I guess I don't see why the 3/4 boring bar holder isn't good enough. They are cheap too.

If you really want a collet holding it then why not an off the shelf MT3 AXA holder and a MT3 3/4" collet.

https://www.shars.com/morse-taper-ho...3-type-153-axa
https://www.shars.com/3-4-3-morse-taper-round-collet