All electrical components go into a waterproof box. Priorities are mechanical sturdyness for shock protection, ease of trouble-shooting, and waterproofness.
Connections through the box's shell are made with waterproof connectors to external sensors and batteries. One option is to epoxy these connectors into place. Another option is to source panel-mount connectors, and use epoxy glue or calking to waterproof around the edges.
On the inside of the box, wires join with the connectors, typically hanging loose off the inside of the box. They break easily. They must be protected. One option is to use heatshrink, which is super-glued in place to the connector and surrounds the wires, reducing the range of motion and stabiliizing them as one unit.
To change the battery adapters from non to waterproof connectors, and then to build waterproof to non cables to go through the box took 10 people hours for 6 battery connectors.
To build 3 DB9 cables to go inside the box took 6 people hours.
Platforms should be used to mount the circuit boards. Multiple layers can be used, typically using a lightweight wood strong enough to withstand the force of plugging/unplugging connectors. Circuit boards should be screwed (or other removable option) to the platforms for ease of removing and inspecting the circuit board.
Box layout can affect signal integrity. Sources of magnetic and electric flux should be kept away from signal lines and receiving antennas. See Noise.
Layout should consider access to all connectors, and tidy running of wiring.
2011 box layout took 2 people hours to design.