The stem is loosened around the steerer tube, the steerer tube must be 4~5 mm shorter than the top of the stem , if not so,

You have to add a A Spacer , that can be added on top of the stem or beneath the stem .

The spacer ring, is that 5mm you seek, not a different top Cap

Just has to not bottom out the underside of the cap against the end of the Fork's steering tube.

the bolt in the Top cap is what does the headset bearing Pre Load adjustment, stem is just another spacer in that situation, once loose,

once that adjustment is done to satisfaction, Tightening the bolts behind the Stem locks that adjustment in place.