The tower consists of Biltema part numbers 88463 (hidden inside), 32323 (air filter) and 88471. I have an 8" fan which I'm also planning to put inside the big adapter piece, so that the motor will always have more than enough airflow, even at low rpm. I just don't have the fan with me right now. Pictures of the installation later.
Update: If you're wondering what this has to do with waterproofing, my thinking is that adding the filter and moving it a little further away would mean less water in the motor. Water entering the filter at standstill should also not find it's way inside the motor nearly as much as it does now. This setup also gives me the option of moving the intake filter somewhere else using some flexible tube.