We've been traveling in the Rockies where internet service is not always available.
My rear bumper on my 66 was in fact hinged along the bumper and opened "backwards". Maybe Airstream engineers were keen on doors opening against the wind instead of with it back in 1966
. My 86 is hinged along the body like a guy might expect. Well, one small dent in the bumper wrecked the alignment of the door. That's the way I purchased the trailer.
So I rigged a simple "stay" for my bumper hatch lid. All the external compartment doors on my trailer are not hinged, they are removable. You unlatch and remove the door to access the compartment. So why not the bumper compartment too? (One of the problems with old Airstreams is lost compartment doors. If you forget to lock them secure, they will wiggle loose and fly into the ditch along the road somewhere. Hinges helped solve this problem.)
So all I did was attach a C channel to the front edge of the compartment, and then attach two rubber draw latches to each side. This holds the bumper compartment lid securely in place, and gets it out of the way when I'm putting old slinky away.
The drain holes are drilled into the belly pan below the bumper storage area, as well as other places in the belly pan. The bumper storage area is not sealed from rain water. It's best to let the water drain out.