Help on lens hood

I just purchased a modified 300/2.8 lens hood. As you can see, it is much shorter than the factory hood. However, the black rubberized ring is not attached. It comes off easily. What would you all recommend exactly to glue this in place?

I would use a few dabs of silicone caulk. It acts as an adhesive, works on most surfaces, and it's reversible in case you want to remove it in the future. Hell, they put aquariums together with silicone, so it will hold.

If you go the silicone caulk route, just make sure you only attach it in a few places. Silicone sticks to nearly everything, but nothing, not even silicone sticks to itself. If you decide later you need to glue it a different way, it won't be possible without FULLY removing the silicone first. Removing an object glued with silicone caulk isn't too difficult, but removing the silicone itself from the item is NOT an easy task, at all!