I also have some discarded 1980s track-lighting fixtures, which used 12-volt incandescent bulbs. I swapped in some LED spotlights, then added a few more to bulb sockets screwed onto a 1x6 pine board.

A red LED, 1.6V, so for 48V that would require 46,4V to be dropped on the resistor. 46,4V * 1mA = 46,4mW of power lost over the resistor (which needs to be 46,4V/1mA=46400Ohm, but lets use 47K for ...