I drove down a dirt road a while back and my truck bed filled with dust. I went back and forth with the tailgate seal options and settled on trying this expanding foam weatherstrip from Home Depot. It is an inch wide and expands to an inch thick. The directions said 3 to 4 hours for expansion but it was fully expanded within an hour at 90F in the shade. The adhesive backing was very sticky and has fiber strands in it.

With the tailgate closed I don't see any light coming in around it. It fills the irregular gaps and didn't tear on the first few opening and closing so hopefully it will hold up. I wish I would have been more conservative at the corners but I cut the side pieces a little longer than needed. I ALMOST had enough left over to seal the front of my tonneau that I have been struggling with. Not sure the couple extra inches would have given up the difference. I don't often drive down long dusty roads so time will tell if it's actually sealed but it looks good for now.

 

There are also gaps around the bed floor in the front if you really want to seal it well.

 

This truck had 1/8" wide cracks in front of both wheel wells. I sealed those with silicone.

I think the gaps in front are for drainage so I will leave them for now.

I don't often drive down long dirt roads. On this last time I had luggage in the back and it was covered but it brushed off easy enough.

 

I have a bed rug that seals most of it. Been looking for something more to seal around the tonneau cover. Looks like it might work well.

 

Did the same thing, about 90% effective. Inexpensive and satisfied. Left the others for drainage as well. Have a drop in.