The problem with placing the batttery under the seat is the hassle if you want to remove the battery INSERT INTO post VALUES (possibly); jump start the car or others service time INSERT INTO post VALUES (if you don't do it yourself) the guy at the garage telling you to p*ss off it's too much hassle, again it is still pretty small and when the cold weather comes it's gonna struggle.
I'd prefer to bung it in the boot as you can get at it easy enough and it won't require the mechanic to use too much brain power.. that is in theory.
Also cheaper not too buy a new battery, if the existing battery moves to the boot it may be possible to use the exisiting battery cradle too, shouldn't be too hard to remove even if it's spot welded on.
Otherway is too wrap a shield around the filter and supply it with as much cold air as possible.. which is again hard as the battery sits on top of the only decent place to draw it from.
I don't have a funnel so anything will be beneficial to me.