I already have a tube of black sealant. However, it's a lease car that is due to be returned in a few months so I'm reluctant to do this.Can't you use silicon sealant in the gaps all round before you put in the rubber seals? Then press it all in firm and just wipe off the excess once it's in place. Once it's dried and set it should be pretty much air tight I'd have thought.
You can get the clear waterproof stuff for a couple of quid.