Designed for the Blackmagic Pocket Cinema Camera but also compatible with most cameras with a 3.5mm audio input, the Hunt A-Box from Camtree inputs two standard 3-pin XLR audio sources and outputs the audio to a single 3.5mm TRRS stereo plug. This passive audio adapter lets you use professional grade microphones to improve the quality of your in-camera audio recording.

The A-Box features a single, integrated 19mm rod port for clamping onto an industry standard 19mm rod. A 15mm adapter bushing is included for mounting on a 15mm rod. Also included is a bolt-on 19mm rod with a 1/4"-20 thread for attaching to any 1/4"-20 mounting screw.
 
https://www.bhphotovideo.com/c/product/1591371-REG/camtree_ch_abox_bmpc_hunt_a_box_for_bmpc.html/overview