This is low-skill, cheap and simple...
Go to 99-cent store, pick up a set of fondue skewers.
Drill appropriate size holes thru top/bottom bumper and nylon end-caps.
Cut two skewers from the set to 6".
Attach a bungee-wrap to each skewer.
Insert your new "end-cap skewers".
Hey now, your end-caps are secure, and that skewer won't come out and puncture a tire behind you. Now be nice and share the left over skewers from the set with your camping friends, or, well, cook something nice on the pit.
+ 2013 Rockwood Windjammer 2809W
+ 2008 GMC Sierra Crew Z71
+ 2002 Ford F-250 7.3L 4X4