Yes, you should add a spacer. A simple one can be fashioned from two sided foam tape. Cut to fit the twisty cap diameter, then cut a hole for the post to stick through. The foam should be just a touch higher than the post in the senter of teh twisty.
Leave the paper on the foam to make it removable. Take the paper off one side to make it permanent. If it's not thick enough, add a second layer of foam tape.
It's not as slick or nice as what Arc provides, but it's easy to do and cheap.
Craft centers also sell compressable foam in sheets.
Daniel