I had some experience with this. I would suggest that putting the box in the water with an attached cable is a bit of a hazard. You could end up losing the whole thing unless you very carefully lay the cable into the stream bed. Access is also an issue – it’s much easier if you can just pull your sensors up from the side.
What we ended up doing was putting the boxes on bridges – however, we did work on private land so you may need permission to do this if you’re working with public. Our biggest problem was that the solar panel kept getting stolen!
For my sensors, I used a 50 mm PVC pipe and wadded pH, O2, conductivity and temperature sensors in there. You’ll need to carefully consider how you deal with the turbidity sensor. I think that sensor will drift A LOT because of deposition/fouling.
Sorry I don’t have any pictures…this is all a few years gone now.
Best of luck!