@Cb83 i done my barn doors and slider paintwork started by 1st drawing a tight line around seal with the doors closed i did't use a edging strip and found its hard to get a clean cut line but it will do for me and stops any condensation forming on the bare paint .
the slider was a bit more awkward with the limited space if i used a blade its cutting into the paintwork... sharp thin scissors but then its tough cutting the carpet with them and hard getting a straight cut its a fiddly Job but was my 1st time ever done lining
all i can say is don't glue round the edges cut over size shut the doors and mark the carpet then carefully cut then glue if you slightly over cut a bit it will stretch and gives you another try
also i used dark Blue carpet was hard to see the marking lines shame there's not a template out there to cut then glue the whole carpet panel in less you can make one
others i seen on U tube have run a rubber trim for the clean edge look of the carpet
but there's no cold spots in the back of my van hope that helps you a bit
Nick