I recently replaced a radio control with a pot from RadioDaze. The original one was splined (grooves parallel to the shaft) on the end where the knob pushes on. My replacement control shaft is just 1/4" round. What is the best way to modify the end of my new shaft so the old knob will push on?

If you split it and bend the split end outward just a bit, it will probably grab just fine, though you could also add a set-screw, since the knob might spin on the shaft and lose position.

You could otherwise locate a knurled shaft and slot it on the inward side to fit a mating key filed onto the original shaft. Then solder into place, and sleeve with a copper/brass sleeve if necessary, for added support.