My plan was to build something like the milling attachment I've seen plans for on the internet kind of like this one here http://www.vintageprojects.com/machi...tach-plans.pdf
Of course it would have to be modified somewhat to make it lay flat and on my Southbend I would replace the compound just like I did with the prototype I built. I only have a 9" lathe so I don't have much room either but I do know the pivot needs to be on the back side of the work.
I do think the unistrut is plenty rigid for this in such a short length and for the lighter cuts we take doing this. The plate I used under that unistrut to mount on the lathe is just an old plate of a piece of farm machinery that's only about a 3/8 thick and the boss I turned to fit the compound mount on the lathe was 2 inch trailer hitch ball and then press fit that into the plate and peened it a few times to make it good and solid. I think the biggest improvement would be a threaded dial feed to advance the tool post for repeatable and accurate cuts which shouldn't be too hard to accomplish