Do you mean these ones or the ones at the front, behind the grille?
In both cases they were already removed from the car when I bought it. These bulkhead ones (or rather the one I had) had a foam gasket to seal to the bulkhead. In some images I have found of bare engine bays, they seem to have been sealed around the edges as well. Whether the sealing is original or an addition, in response to engine smells entering the cabin, I don't know, but it seems like it would be difficult to maintain a good seal with foam alone, given the number and spacing of screw fixings. I haven't decided, yet, how I will seal mine - possibly with butyl rubber strip.
The front ones are a mystery. I haven't spent a long time experimenting, but it seems almost impossible to get them back in. I may well resort to cutting the brackets off and making new ones that can be screwed into position after fitting the ducts.