I don't know about you, but we lose these things all the time. We use our track lighting for mobile display at a craft fair booth, and the end caps are always going missing. And they're actually pretty important. If you start installing lights on a track without an end, sometimes it will have the actual track insert sticking out one end without you noticing. When it comes time to insert the power plug, either the plug will drive out the tracks (if you have no lights installed yet) or it will sometimes not even reach the tracks to power them if they slid out the end before you started installing lights. 

This is a simple replacement for those. Obviously, you probably want to print it in white or a color matching your track lights. 

Requires a small sheet metal screw to hold it in place: I use a #5 x ½" flat head sheet metal screw. A 2.9 or 3.3mm, 10-12mm long sheet metal self tapping screw should work too. Flat head or oval head won't matter.