Cockpits and high-detail interiors are super time-consuming, considering you need to use ultra-high amounts of parts to get a simple circle, or half circle. A "negate-space" function would work like this:

1. Build the overall shape of the object you will be subtracting from.

2. place another block into the object, in the desired form of the negative space you would like

3. Press a hotkey (CTRL + N?) to subtract space from the block.

Hitbox does not need to change, only the mesh. Could be made more user-friendly by adding a way to select a specific part to subtract from.
This could add a much higher level of detail to most aircraft in the game, and cause more uploads to the SimplePlanes website.

Could be used for numerous things from realistic engines, to cockpits, passenger fuselages, cargo/weapon bays.