Where is the schematic for the boards that you are selling? If that board has two regulators in parallel (like the original DC SSR boards), there is another inexpensive way.
First, do not install the 5V regulators that are mentioned in the BOM.
Second, take a 5.1V zener diode and install it in place of the LM78L05 regulator (anode to the ground pin of the 'L05 footprint, cathode to the output pin of the 'L05 footprint). This diode might be a 1N5231A or 1N4733A, the exact part to be selected on the basis of working well at a low current level. This diode would not lay flat on the board, rather standing on end, so it might be a good idea to use a short length of shrink-wrap tubing to protect it.
Third, take a 1.5K,1W resistor (selected for pushing 20mA through the zener with a 36V input level) and install it between the input and output pins of the LM7805 regulator. This part also ends up standing on end, so it should also be protected with a short length of shrink-wrap tubing.
This potential solution should be prototyped of course. Its advantages are that it is relatively cheap, and does not require any cuts or jumpers to the board. The disadvantage is that the regulator is always dissipating 720mW of power between the diode and the resistor described above (when powered with 36V), and the value of the resistor would need to be changed if the input voltage is radically different from 36V.