Merkins are an odd kind of wig... you can investigate what they are particularly, should you be so inclined (not for the faint of heart and very conservative).
So a merkinabox is a box with what looks like a merkin inside it. Don't worry; they're "fresh" pieces of recycled fur. The box can be any shape or size, and can be made to measure. If you're in dire need for a merkinabox immediately, your wish can be fulfilled; I have several in stock at the moment.