Don't overthink the power supply or re-invent. There are many LDO devices that are very small and contain all the protection you need. check out www.maxim-ic.com or www.ti.com. Many of these companies will send you free samples.
Look for something that will handle 40V-60V input and regulate the 5V output. Also make sure it has reverse voltage protection and overtemperature protection.
Good pointers, I'll go look at that. I got a 20v regulator but I was still going to put in the diodes and resistor just to spread the heat around. I'm not too nervous about shedding 1/2 watt in a resistor and another watt in the regulator but all this stuff IS right near my gas tank!