When I installed by FF on my Scamp
using butyl tape under the flange and snap caps over the screws on the roof. Turned out well. I did add a small bead of Sikaflex 221 around the vent once installed. Worked well.
Only thing I would do differently would be to use a Deluxe Maxxfan
rather than a FF. That way you will be able to leave the vent open in the rain without the need for a separate vent cover. Recently installed a Deluxe Maxxfan in my current trailer over the bed area and very happy with it - to me it is a far better system than the deluxe FF with rain sensor/remote etc that was already on the trailer at the other end of it. Find the FF is louder and has a more complicated auto control system & needs to be shut if its raining out.