I mounted mine in the cabinet under the sink and next to my shower. The vent is standard 4 x 12 x 6 HVAC right angle boot (sometimes called a torpedo) which was close in size to the outside of the tankless, crimped for a tight fit, pop riveted, then sealed with hightemp aluminum duct tape. The flue was a standard 6" double wall but I had added a 6 x 5 reducer to the boot making it a triple wall where it passes through.
The tankless stands off the wall about 3 inches on the closest side. I had added a sheet of aluminum on the other, not as a precaution but to cover a hole, but hasn't proven to be necessary so did not bother to do the same on the close side. A small operable louvered metal register vent would be better for the outside exhaust than a plastic dryer vent.
On the lowest setting it is fine for the shower without a cold water line and mixing valve but I did replace the plastic hose that came with the shower head with a section of the braided steel toilet connector after the plastic melted.
More pics and details on page 3 of my Log Cabin build journal.
