I have a 9.9 Mercury 4 stroke motor, and I lay it down on its handle against the rear of my pickup box forward of the 5th wheel hitch. It has never leaked oil in the truck. Check with your motor manufacturer, I think you will find that it can be laid down on the handle. The handle on my motor even unlocks and folds down toward the propeller for a smaller footprint when travelling. I use a bike wire lock to lock it to the 5th wheel hitch inside the box.
2009 GMC Sierra SLT Crew Cab 4x4 Diesel
2010 Rockwood 8280SS
Camping 2012: 40 nights