Should you find yourself with a loosey-goosey gas cap on your R50 or R53; the probable culprit is a broken “white” fastener, and/or loose closing spring. To my knowledge, you have to buy the entire gas cap hinge assembly (over $145) to correct. The “white” fasteners are not available separately.
An alternative fix is to purchase a used hinge assembly or part out from a salvage yard.
The gas cap hinge assembly is not that difficult to replace but requires patience and a few tricks.

 

The wire commonly pops out of the white thing too!

 

Hasn't happened to my 20y/o R53...yet.
But good to know.
Thank you for posting!