What is the most interesting catch can you have seen on a car?
I have made some out of those cheap stainless water bottles like that, but seriously I can order something magnitudes nicer off temu for like $10. Not worth the effort to make them at that price. I just order several in and have them on hand ready to put on a project.
I had a coolant tank made from a metal paint thinner can for a while. Worked bretty good. I have some empty Motul 300V cans now, they are 2 quarts and square, I may use one of those next time.