Time: 2 Hours
Another modification I’m doing to my RV is to move the oil filter from the stock left side of the rear baffles to the right side on the engine mount. This allows for a much larger cooler to help with cooling my larger HP engine. To get the air from the engine area to the cooler is via a hole cut into the right side aft baffle and a fiberglass plenum. The cooler gets mounted to the engine mount via a two piece bracket and locks the cooler into a specific spot based on the geometry of the engine mount. I had the cooler temporary mounted to the brackets and used it with the plenum to get the initial cut line after I trimmed the side of the plenum to fit the cooler. 
I returned the rear baffle and worked the plenum until I had a rough 1/4″ gap, this will get sealed with a rubber gasket eventually. To cut the aft side of the baffle ShowPlanes gives you a doubler with the hole steady cut into it. Unfortunately the hole in their doubler was too high for my plenum, not sure why but not a big deal…I’ll just make a new one.
The fit was perfect so I laid out a rivet line at the top, left and bottom. The right side could be match drilled to the baffle rivets.
After I had all the holes drilled and deburred I removed the material from the baffle.
After I was happy with all the edges on the inside I trimmed the top, bottom and right sides to make a nice fit. I dimpled all the common holes between the doubler and baffle. I also primed the surface of the double and riveted the holes except the row that are common to the side baffle as those will get done later.
This was one of those items that works in conjunction with the baffles that needed to get done while working on them.