I suspect that you might have the best results using a Sram mtn FD, setting the height as if there were a 48 or 50t in the outer position. That will locate the shift ramp shaped into the cage's inner plate at the right height for shifting up off the 24t. The FD won't know or care that there's no outer ring there, but make sure that the dummy ring or bashguard on the outside is small enough for the cage to clear it.