An enclosure is not packaging. It sets your thermal ceiling, decides how long installation takes and determines whether anyone can service the unit without removing it.
Design for cabinet ambient, not room ambient
The inside of a closed control cabinet sits well above the room it stands in, and it sits higher still near the top where equipment is usually mounted. Derate your component selection against the cabinet temperature you expect on the worst day of the year, not against a bench measurement.
Mounting decides installation time
DIN rail mounting is the difference between a clip and a drilling job. If your target position is inside an existing cabinet, rail compatibility is not a nice detail, it is what makes the install a short task rather than a scheduled intervention.
Put the connectors where the wires already are
Field wiring enters cabinets from a consistent direction. Connectors placed on the wrong face force loops of cable that get pinched by the door. Group the connectors by the trade that touches them: power and machine wiring on one edge, network and service ports on another.
Label at the connector, not in the manual
The manual will not be there at two in the morning. Screen printing the function next to each terminal costs nothing at manufacture and prevents an entire category of installation error.
Passive first
A fan is a moving part, an ingress path and a maintenance item. In a dusty environment it is also a filter that nobody will clean. Design the thermal path around conduction and surface area first, and treat active cooling as an admission that the compute budget needs revisiting.
Leave room for the second revision
Prototype enclosures should have space for the connector you have not specified yet. The cost of a slightly larger box during development is far lower than the cost of a new tool once you have committed.