I have recently decided to upgrade my old van by installing a few 5V/3A sockets using a 100W 12V-to-5V downstep buck that has no reverse-polarity protection. A little bit of searching here and elsewhere has led me to understand that I can facilitate that protection with either a diode, at the cost of a significant voltage drop and risk of reverse leakage, or a MOSFET set before the buck circuit. I've tried looking for appropriate p-FETs but have only found 7812s @1A/1.5A and variable I/O FETs that I understand would require additional components to configure.
Which of these techniques would be more appropriate to use for my purpose? If I use a p-FET, what should I be looking for to find the most appropriate one?
Thank you for your time.