Drafted in the finest of CAD programs, TinkerCAD (Which is why somehow the hole is slightly off center despite being aligned in TinkerCAD, I swear, still can't figure that out), this replacement cap for the Bissell 1867 Steam Mop is simple and effective.

Probably just best to print it with 100% infill, mostly due to the thinness of the top, but your mileage may vary depending on how rough you are and what material you choose. The original part had some fancier bevels and knurling that I may or may not add in a later revision, though they aren't needed.

I printed in it in Monoprice PLA Plus (relabaled eSun PLA+). Any material should be suitable as it is a low-stress part and heat from mop doesn't travel up that high.