Ok, if you have a Ford truck you are prolly miss'n one or more of these silly caps. I run a 2005 F250 Superduty but these should fit F150 as well as other models around that time frame. I designed them so they actually pop in instead of gingerly sit on the lip surround and just waiting for that next pot-hole impact to fly out.

SketchUp file included along with the stl file for 2 caps.