What Brasco says brings me on an idea...If you have the budget and time to make some mould and need to get many parts out of it you could thermoform plastics...
Thats how they make jacuzzis and big parts for tractors and so on...
If you're able to make CAD models of you parts you could CNC the parts so you have your moulds easily...
this can maybe get a bit expensive but is relatively easy to do...