Have you thought of an oil undercoating. I make this homemade grease undercoating and it works fantastic at slowing down or stopping rust. Btw I have a 98 Ford ranger and the entire underside of the vehicle has been coated for 3 years and it's working fantastically.
Can you give us the recipe for your homemade undercoating?
I made a homemade undercoating out of wax from a toilet bowl wax ring*...bar and chain oil (motor oil works too) and mineral spirits. It's a variation of a recipe that I read about on this site. It doesn't spray well because of the wax so I brush it liberally on the underside of the vehicle....seems to work pretty well.
* The toilet ring wax was the type recommended by the original thread (as opposed to candle wax)....I think he called it parafinic wax
PS: If you use POR-15....only buy the amount you can use soon....it does work well but I've never
been able to store leftover amounts without it hardening.