How to protect againt surges if working and maximum voltage are too close
Well first of all I'd make sure I bought a 48V supply that was guaranteed NOT to go out of some tolerance range that you can accept.

Otherwise you would be better to add a circuit that shuts off the power to your regulator if the source goes above the range you deem acceptable. If the supply is going to fail out of spec, a simple TVS or Zener will not last long.

The design in this answer to another question may be more to your liking. The circuit there is of course for 5V but if you understand it, you can modify it to whatever voltage you need to limit to. Watch you do not drop the gate of the MOSFET below its limit though.