For me personally I would cut a hole big enough for insulation to insulate the duct sleeve all the way to the air outlet/register. Don't know that specific heater but most are made for a good bit of ductwork to be connected to them. With a short run of ductwork the metal is going to be very hot. Not saying it will start a fire but long term exposure(winter time) if touching wood it could happen.
So use a good insulation to seperate everything.
My concern was the actual install of the unit itself especially if it is running while on the road and the heater exhaust and intake in the proper orientation?
Heater won't burn right and if a safety switch/switches fail then you have a time bomb?
Sorry, I do a lot of govt fuel/oil,steam,natural gas and are dabbling in bio-diesel boiler installs so safety is first in my head but proper install is second.
Proper air in and out of any diesel fired appliance is what makes it run as designed to much in and not enough out makes it inefficient to much out and not enough in is the same.
What I am trying to say is just cause it works good setting in your driveway doesn't men that it will work like that while going down the road?
The intake needs clean air in and the exhaust needs an un-interrupted outlet for perfect performance.
That's my opinion?
Look at a super saver turkey heater xl install.
I can put one of these on the 1st floor of. Building and heat the 2nd and 3rd and one on the 4th floor to heat 4th,5th and 6th and so on everything radiates up in a concrete construction world but proper install still calls for fresh air and uninterrupted exhaust?