There fairly small and there is plenty of room in the average engine bay but you'll need to consider what else you have in there, eber, leisure battery etc.......
I personnally have the roof mount kit that hopefully will be fitted this week, if the snow ever stops, horizontal and vertical it doesnt matter when specifying as it is essentially the same, however the roof will fit in all 3 orientations, its just that the meter is positioned in relation to the pick up pipe so when suspended from the roof the meter can be easily read.
The main issue is really what substance you want to use, theres only 2 to pick really AFFF or dry powder, both very messy and effective in an engine bay, if the dry powder is your medicine of choice then remember to shake up the bottle every now and again prefferably part of your regular maintenance.
Whilst your at it change all your fuel lines, pointless installing it if your gonna set it off within a few weeks.
If you need contact details or more info give me a call, I cant see another group buy going ahead so you'll not get the benefit of 40% discount but even at full cost its worth the piece of mind.......
I was working on a tank to fit into the spare wheel well but the cost for non standard bottle just wouldnt be cos effective sadly....