When I bought mine they had a Corolla flipped on its side in the showroom with the underside exposed showing a demonstration of their rubber-like undercoating compound. I made them throw it in for "free" as part of the deal. Mind you, the 4Runner had sat on the lot all winter and did have 200km on it when I bought it, but they did spray/clean the undercarriage first before applying. It looks pretty good and the whole undercarriage is black. I pay $30 once per year to have it inspected and touched up as needed. It smells really good afterward lol. Small price for a little extra piece of mind.

Maybe you can investigate something like Krown. Also, Canadian Tire's product rated really well in a military study, but no idea how well it works.