I had an old Colchester lathe top slide that I have converted to a vertical milling slide on my SB.
The slide was shortened and the face machined square to the sides and two tapped holes for a fence if used.
T slots are machined in the face, stopping before breaking through to the inner casting recess, the slides were then refurbished to a nice fit, all that was left to do was make a sturdy bracket for the boring table from half inch M/S plate (forgive the welding this was done years ago with a borrowed stick welder, I am better at it now).
Two full length T nuts stay with the slide so it slips in place on the boring table in seconds, two small tabs on the rear edge align it square to the lathe mandrel.
A old Record vice is usually in place which has been machined to improve it.
I have used this slide a lot over the years and have done some serious milling with it, its nice and heavy and keeps overhang to a minimum so is very stable in use, better I think than some commercial offerings.