Spray Foam For Sealing Holes?

I have a space where the door attaches to the truck that ALWAYS fills with sand and dirt after offload trips. Checked it out and there is a pizza slice shaped opening there. At the bottom it's about 1.5" and the hole is about 3" tall. No wonder debris gets thrown in there from the front tires.

I'm thinking of getting a can of that spray foam that fills gaps you buy at the hardware store. Has anyone used this stuff on their trucks?

Also contemplated filling the boxed in sections of the frame to keep mud and water out. Bad idea?

 

I was considering using some sort of expanding foam to fill that gap. Haven't done it yet tho.

 

If water does happen to get into the spots you've filled, it will sit there and could cause more of a rust issue.