I’ve just had a Webasto fitted, using the factory vents (like Deaky’s) and the factory mounts for the heater underneath. It’s a really tidy job.
While it’s a Kombi, I have Caravelle floor and rails, so this is pretty much the only option that would work for me, as passenger seat houses the sub and 2nd battery is under the drivers seat.
If you are in the South, the guys at Absolut5 are the people to speak to.