There are two containers connected with a thin sheet of plastic for keeping a pair of contact lenses. Should we call them (or it)
Contact lenses cases
or
A contact lenses case
or
Contact lens cases
or
A contact lens case
There are two containers connected with a thin sheet of plastic for keeping a pair of contact lenses. Should we call them (or it)
Contact lenses cases
or
A contact lenses case
or
Contact lens cases
or
A contact lens case
For a single container to hold multiple items, both container and contained are singular. A matchbox, a briefcase, a bookshelf, a notebook, a sock drawer.
Glasses case is widely used, maybe because glass case would be confusing. Some people are not comfortable with this usage and say spectacle case instead.
The correct term is therefore
a contact lens case