Sure Weps, no problem
Once the downtube is cut to rough length, I place two long sloping wedges of tool steel (fat on one end, skinny on the other, about 6" high) into the mill vice and insert the tube between them.
The end of the tube to be ovalized is nestled between the thick ends of the wedges. The vice is slowly drawn together, squeezing the tube end, past the point of ovalization by about 10% and then the pressure is reduced, allowing the tubing to open back up a bit. During this process, the skinny ends of the wedge travel in against the tube and the tapering shape keeps the tube from crimping.
To ovalaize the other end, simply swap ends, rotate 90 degrees on the axis, center with dial indicator and repeat.
Make sense? No real magic wand, just good ol' american ingenuity