Gas cap lid pulls open in car wash

Then don't go there. Find other place better.. Such as touchless. Or wash it at home.

There are several type of wash places, such as touchless, brushes, etc.

Or pay extra for full service car wash which they can clean inside and out, also they can dry your truck with towels in their hands.

 

I'd try magnets. I run mine through a non touchless twice a week and mine has never done that. My passenger mirror blows outward but that is it.

 

Gas cover is plastic, magnets aren't an option.

 

Glue or double sided tape?

 

I know it's not an option but what if you ran the truck in reverse through the car wash?

 

Right. Get a set. Each side would stick to the plastic.