For this build we are installing a T-vent style vent window. On stock Sprinter cargo vans without a window on the slider door there is a blind spot so this really helps when driving. A big factor when camping is having that bit of extra air flow by opening the vent on the window.
We start by cutting the area for the window and then trimming any areas that are not smooth.
The window simply clamps together and has a very thick gasket for a nice waterproof seal.
The vent also has a nice little screen to keep bugs out when opened.