resealing

Good afternoon, gentlemen. I need to replace the (missing) plastic waterproof door membrane on my R129. Polythene builders' sheeting is not a problem for the membrane, but I've no idea which sealant to use to stick it in place. Does anybody have a recommendation?

I changed my plugs and wires and I discovered that I have a small oil leak on the cover that helps recirculate oil on top of each of the valve covers. http://i141.photobucket.com/albums/r69/TruTaing/Resealing%20recirculating%20covers/IMG_0447.jpg There is no gasket that can be replaced...