Aah okay, no exactly neither the box nor the contacts have a particularly high IP rating. So choosing a good place to mount the system away from rain and debris is a good idea. The kit I sell at Tindie comes with simple DB15 connectors with solder connection, they have a simple model of strain relief that you should use.
Regarding connection of paired injectors, I see no problem because they have double pins in the IDC40 connector. See attached picture, In contact A there are double pins for the injectors.
Regarding how much power you can draw, I refer to the video Josh has uploaded on this topic, which I link to in a previous post
I'm building three more kits, which I will soon post on Tindie.