Hi there,
This is Bogdan from Romania.
I'm a mechanical engineer, specialized in the industrial robotics, 3D modeler and designer.
I have over 15 years work experience, designing, prototyping and launching different devices/systems across multiple industries.
I could really help with your project, but there are a lot of questions to be addressed...
1 - as you pointed, out there are a lot of models and systems, each one with pros and cons, and amending one/any of them will be a waste of time (we could discuss days on any of them..)
2 - ip66/67 is reachable, but for this is necessary to have another way to plug in the cords/plugs. in any of these systems/models, when opening the lid the protection disappeared..
3 - the locking mechanism if it is mechanical, sooner or later will lose the seal and will allows intrusion of the water. the best/only solution is to use a magnetic latch
4 - i'm not fully accommodate with low-tech socket. i'm not saying to use state of the art but neither low-tech
5 - the cost per unit of less than 200 per unit is a relative parameters and depends on a number of factors, but basically is reachable
For any other questions, you may open a chat room here and we'll discuss about the details.
Regards
Bogdan