RE: What are you doing to
The mesh ones are available at a lot of accessory stores, you can likely find them on line too.
There are two types:
The first is made out of nylon that's more or less a net. It's also more or less useless if you haul a lot of stuff.
The second is a hard gate made out of metal screening. It has a latch just like your present gate. They are available in either fifth wheel (dip in the centre for the gooseneck) or regular configurations. These are good for hauling everything but topsoil and other fine materials.
The idea of both is to let the air pass through.
I still find the best method is just to drop the tailgate. You can only do that when your truck is empty though, or you will get a ticket. If you have the back full of bales it doesn't matter if the gate is up though because the bales will block the air from the box.
Other options are caps and tonneau covers, which pass the air over the box. If you are hauling hay, they would be a serious pain in the ass though.