We put pieces of that interlocking EVA foam gym floor tile in that gap. We can fit a stack of two on either side of the truck. It's just a good place to stick these and in our truck, a stack of two foam tiles wedges in enough that they stayed there on a 3400 mile trip. Years ago, my wife found 20 or so of these tiles for sale on craigslist. I can't tell you how many things we use them for around the house, from gardening knee pads to protective padding for delicate items. When we are camping, they are nice to have if you need to get under the truck for something, to place at the bottom of our camper steps if it is muddy or dirty (we also bring a lightweight boot scrubbing station thing that we put on top of the foam tiles below the steps), or to place under the leveling blocks on hard surfaces so they don't skid around. Since we have a stack of these at home and paid next to nothing, we don't care if they get chewed up.
Just make sure to clean any dirt/grit off of them before sticking them in between the bed and camper. The tops of the tiles move around a bit when traveling at speed, and you wouldn't want to wear into any paint or window glass. They do make a little bit of muffled sound when driving fast, but not enough to bug me, someone who is obsessed with eliminating unnecessary noises.
And judging by you having 2" gap there and me only having enough to wedge two foam tiles, I'm guessing FWC uses the large bumpers to accommodate the various designs of different truck beds. I have a 2018 Ram and there is a significant difference in the location of the top of the front bed rail and the bottom of it. If I trimmed 3/4" off the bumpers, the camper woud probably be touching the top of the front bed rail. So keep that in mind when trimming if the camper might end up on a different truck in the future.