Buzz, when I purchase a new cover for the rear truck bed, I make them figure it out with the purchase, yes they know ahead of time, water, dust and all other items. This has worked for me and usually no real added costs to speak of.
I’ll have to take some pics of what was done to help you.
This has been an ongoing issue for us while pheasant hunting. Have tried rubber weather strips and spray-foam which have helped some but still get plenty of dust on everything back there..
I silicone all the bed gaps along the sides and front on every new truck I purchase. !I also put on a tailgate seal and install a bedrug. It helps keep most of the dust out.