There's three different "correct" ones.
For the earlier chrome lens, there's a round one, which is marked for 50/1.8, 35/2.8, and 35/3.2 lenses. There's a rectangular one, which is vented for the viewfinder, typically marked for the same three lenses. It is rarer than the round one.
For the later black/chrome lens, there's the S-42 hood.
All of these hoods have Series VI sized threads on the back of them, and hopefully will come with a proper adapter.
The round one is usually found/used with a 40mm screw-in to Series VI adapter. The rectangular one is usually found/used with a 42mm slip-on to Series VI adapter. The S-42 normally comes with a 42mm screw-clamp to Series VI adapter.
I strongly prefer the rectangular one, since it doesn't block the viewfinder at all on a bottom-load Canon, or on a later Canon like the 7s.