A unique product, the Op/Tech USA Filter Cap will allow you to stack a filter without having to remove the lens cover, and is held in place by the filter of your choice. As it's positioned behind the filter, it enables other filters to be stacked on top without causing any vignetting. The cap than closes and covers the filter screwed onto the front of the lens.
It also provides impact protection to the rim of the filter and lens which is often a common problem amongst avid photographers