I get some 12" wide flashing, about 18" long. Put a creased lip on the back edge and make a spring curl on the front and just snap it over the hitch plate. Does the trick for me and doesn't take up much space.
I grease my king pin and hitch plate, and the above cover keeps the plate clean when the trailer is detached... and reduces accidental contact when I'm crawling around in the pickup bed.
Originally I used a nice piece of brass flashing (as an old navy hand, gotta like polished brass). But lost it and could find only aluminum which is available by the foot from rolls. Have run thru a couple of aluminum covers since.
Not fancy but does the trick.