According to Oxford
The tonneau is:
n.
the part of a motor car occupied by the back seats, esp. in an open car.
and a tonneau cover is:
a removable flexible cover for the passenger seats in an open car, boat, etc., when they are not in use.
It is the name in the product literature for the thing that the thread's OP is looking for. Enforcer's photo is of the mounts for the tonneau cover, which, in Canada, comes with every MO.
We even pay less (if you calculate the exchange) but we get less discount. Does that mean we pay more?
Anyway, tonneau cover is probably what Oxford says, what Nissan says, and the thing for a truck... or anything that covers a hole.
"Honey, did you wash my tonneau covers this week?"