I would think the installation instructions for the unit would specify the layout and materials required for a proper setup. If not, call the manufacturer.

Code for horizontal vent runs of a power vented heater (non-condensing) is category 3 venting. Single or double wall are acceptable as long as you follow the vent manufacturer's guidelines. Had to do this recently for a Mr. heater Big Maxx unit. I ended up getting this category 3 vent kit because the Mr. heater one was too expensive.