Well in the uk the law states , must be concealed in a way that it can nto be fired. So ether of what you mention is
fine. The most common however would be a slip.
I do ocasionaly come across members of my club who use a case, the majority use a slip.
A slip can be down to personal preferance, and made from a selection of materials, cost can be from as littl as £20 to £140 for a hand stitched english leather slip.