If you only need one than the cheapest is find a local machinest and say "make a case to fit" It will be all manual and designed on the mill/lathe except where experience says "better draw this part up first" (one in a while they will scrap a part because it doesn't fit, but overall still cheap). If you need 5-100 then draw it up in CAD and have a machinist (need not be local) throw it on their CNC machines. If you need 1000 or more then design an injection process. Where I put exact numbers that is because the overhead vs efficiency of the process makes this best, where I didn't put anything it is a judgement call, it isn't clear when exactly ones process should be abandoned for the next. (even then sometimes I'm wrong, but for discussion I'm close enough - if you are doing real world work consult a real manufacturing engineer)

Injection molding is great for making a lot of parts, but the cost of designing a working mold means the upfront costs are a lot higher and if you cannot reuse that mold enough to spread that cost across many parts it isn't worth doing.