4-6 months is called a "storage". In storage you do need a maintainer, hardly any news.
2.5W is enough for maintenance without any loads (like in my old truck where I disconnect the battery).
5W "might" be enough with some loads like computer and radio standby, or it might not. They come a bit cheaper though:
5W.Without any loads (like when you disconnect the battery) with 5W I would be worried about overcharging, but not much. These are very simple chargers, with night diode but without charge control.
I keep it on the roof or on the hood, tied with a shoelace to something. There is enough flex in the door rubber trim to put the cable through and close the door (if plugged into cig socket). Or - clip it to battery and run cable under the hood. Or put it on the dash, but there will be more shading there. If plugged into cig socket, make sure that it's On, because in some cars it's Off when you turn the ignition Off.
Vertical placement on the window is about same good in winter as flat on the hood, as the optimal angle in winter is close to 45 deg. In summer it should better be horizontal.
