Noga
Mirrorless cameras are small, light, and can get into spaces traditional cinema and ENG cameras just can't. However, mirrorless cameras are not meant to work on a film set all day long—it just isn't in their ergonomics. This is especially true if you are planning on handholding a camera all day, or working with accessories such as follow focus units, zoom motors, or external monitors.
Build Your Own or Pre-Made
You can build your own support rig, if you like. Head out to your machine shop and have at it. But, most likely, you lack the milling