This is a cap to protect the mattress from screw poking up from bed support slides on pop up camper. mine is a Fleetwood Avalon. this is where the bolts stick up from the bed support brackets under the mattresses. 

this is for a 10-24 thread, you may need to clean up the thread after printing depending on your print settings.

I printed mine from PLA with 15 percent infill, 2 wall thickness, standard purse .4 nozzle, and no supports.