To be honest the cowl is not really meant to be used as a vent. The cowl is in the box and I leave it there just in case anyone has a use for it. The flue is there as an optional extra for people to direct the fumes to where they want them to go, whether it is to the back door, or up to a proper vent in the van. I did however modify a cowl to use as a vent in someone's van, it was not very professional as it was welded to a metal plate, which in turn screwed on over a hole in the van roof. It worked well and the unprofessional ness was not seen on the van roof. Now there is no way I am going to make them available to get slaughtered on here, until I can make them a lot better looking. I will though when I get the time.
Peter