Well, it's a cheap, Chinese 24 VDC switching supply so just trying to
avoid spiking at start-up. Is it as simple as a 24 V zener across the
input (+ to -)? If so, I have a few of those here.

Just that simple, yes. Not sure how much current goes into this device, but you want the diode to be rated at least 2x that current so it does not fail under load.

Peter Wieck
Melrose Park, PA