This mount is designed to attach an MKESC 75100 V2 to my Trek 4500 mountain bike. It is far from perfect (not watertight, scratches the frame) but it does the job and can be used as a starting point for designing your own version (.f3d file included). The housing clamps onto the frame tube of your mountainbike. I recommend adding some tyraps to keep it in place.

The housing and tray can both be printed without supports. The model is mostly parametric so adjusting things like the tube diameter shouldn't be too difficult.