When the main gear is retracted, the opening (most of it) is covered with a door. This door has to be cut to fit. Here’s the door before cutting. (The pencil line are reference lines for cutting )

Here’s the door after cutting. The tongue depressors are glued to the door to keep it flush with the surface.

I looked at that bottom opening and thought about a second door to close that off but decided it would too much work for not enough gain. 

Then the back side of the doors have to be strengthened/stiffened. This is done by gluing 1/4″ foam to the inside, beveling the edges and glassing over it with 2 layers of BID.