When I purchased a new hatch seal, I was fully aware of the fact that it had to be installed using a sealant in the clamp-on channel which attaches to the body. This is also what the website of kyle, only 944.com makes very clear. I was a bit lazy and gave it a try without sealant. To no surprise water ingressed into the boot, due to the capillary effect of water creeping virtually everywhere.
So I bought a cartridge with an elastic sealer and applied it today:
While I was in the garage with the heater working, I also applied a light tint to the back-up lights: