Motopsychojdn
Illustrious
@Mjh, sorry fordelay mate, busy yesterday 
how about grabbing a plastic/fibreglass cover thing like say, http://www.ebay.co.uk/itm/TRUMA-ULTRASTORE-WATER-HEATER-GRILL-COVER-POST-07-2006-IN-IVORY-P-NO-70122-03-/170886061120?pt=UK_Campervan_Caravan_Accessories&hash=item27c99a5c40
trim to the width of your case roof if neccesary and glue/blue tac/weld into place so its sealed front and sides so that airflow is directed to the rear of the pc?
paint/decorate to suit as well ofc
that will alleviate you of any unwanted warm drafts I reckon
@Jaber, fan down mate, especially if you have grill/venting in the bottom of the case
Moto

how about grabbing a plastic/fibreglass cover thing like say, http://www.ebay.co.uk/itm/TRUMA-ULTRASTORE-WATER-HEATER-GRILL-COVER-POST-07-2006-IN-IVORY-P-NO-70122-03-/170886061120?pt=UK_Campervan_Caravan_Accessories&hash=item27c99a5c40
trim to the width of your case roof if neccesary and glue/blue tac/weld into place so its sealed front and sides so that airflow is directed to the rear of the pc?
paint/decorate to suit as well ofc

that will alleviate you of any unwanted warm drafts I reckon
@Jaber, fan down mate, especially if you have grill/venting in the bottom of the case
Moto