Jon,
The "custom part" may be nothing but a hose reducer. Typical RV water tanks use 1-1/2" hoses. Other applications use other sizes. You might find all the parts and adapt them, but anytime you go down a rabbit hole like this, expect to spend time and money. You will probably end up with a box of unused parts, and a filler that is just what you want. I did this years ago on an electrical port. I think I ended up spending $300 to create something I couldn't find anywhere. One option is to use our inner parts, and just find a metal door to cover it. The neck and cap that we sell is very good quality, and no worry about water quality from unsafe materials. The outer door is removable and replaceable. That is one big advantage of the stuff we sell. We have parts, and we get things out fast.