Fender washers are just large-diameter flat washers used to attach small screws or bolts to soft or thin materials, or to oversized holes where the screw or bolt heads might slip through or bind in the gap.
The plastic strips in the rubber won't accomodate holes for anything much larger than about a #8 sheet-metal screw. But the slots in the aluminum bumper are pretty large for those screws. So fender washers are needed. You should be able to find some 3/4" or 1" diameter washers to fit a #8 screw in any hardware store.
For my initial 'fix, because I didn't want to waste time running to a hardware store, I used what I happened to find on my workbench - a few #8 x 3/4" plated steel Philips sheet-metal screws and a 1"x 3" flat bit of galvanized sheet-steel to cut & drill my own fender-washers. I already had the right end of the bumper-rubber hanging loose from the bumper, when I drilled the holes in the plastic strips. I was careful not to drill the holes all the way through the outside of the rubber. After drilling, I pre-threaded those holes with the screws to make it easier to re-install the hardware from under/behind the bumper. I used a stubby Philips screwdriver so I didn't need to remove the bumper.
Since this fix is in a wet, dirty location, I recommend using stainless screws & washers, just in case, for whatever reason you might need to remove those strips in the future.
Like I may have to if any more of those plastic hooks break off later.
Now that the drilling & fitting has been done, I plan an upgrade to stainless as it will be pretty easy now to swap the hardware, one-at-a-time, without having to remove the strips.

Happy Motoring, Mark

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DrDKW