I wouldn't mount it flat. That would restrict the air flow/cooling too much.
Most residential mini-splits also do. Good job of heating thus eliminating the need of a furnace and one less appliance that needs propane. If you seldom boondock, and/or are willing to heat water with electricity, you have eliminated another need for propane. Cooking with electricity is a "no brainer".
The point of that discussion is, with no propane, you now have a good mounting location for the mini-split compressor !