Looks like a screw cap. Should be available at any hardware store or big box home center --
e.g., https://www.lowes.com/pd/Hillman-1-4-in-x-1-4-in-White-Plastic-End-Cap/3012662
Quoted from delt31:Thanks guys -
btw noticed these white covers on the sling screws in this video - do you know where he got them from?
https://www.youtube.com/watch?time_continue=337&v=p6paATc3lcM