you could try a contact glue like bostick or the likes. cheapest way to get some is to drive through a chav estate and look fer one passed out then nick his glue bag
I used automotive seam sealer from a mastic gun,it sticks like poo to a blanket but there are probably cheaper ways of doing it.I figured that it would keep any water from getting between the seals and the rust-prone bulkhead.
I used ordinary silicone sealant (clear) and it worked fine. The new seals will look like they are a poor fit to begin with but if you tighten the vents down completely after applying the sillicone they will eventually settle down and the edge of the vent will seat itself in the new seal.