those connectors you used are for panel mounting. typically this means a metal enclosure. It could be plastic but it has to be relatively thin which is not that strong.
I would try to find a small tin that is the right shape and can be worked with simple tools. Another option is mounting everything to a small piece of plywood or balsa wood. And then mounting that inside of a larger box with a pass through slot for the connecting wires.
2 for sure! 3D Printing has its uses but, as I believe sheep is alluding to, it's not always the best solution.
I love designing for 3D printers and if you go that route, I'd recommend Fusion 360. Like anything else, it requires practice. Previous drafting experience is helpful, but not required! If you can imagine it, you can bring it to life! If you'd like links to some resources, @nonosto, let me know.