You need to talk to a tooling supplier, for 300 pieces for something that large you would probably go with a short run aluminum tool, and if feasible use rapid prototyped parts where possible.
Given the shape and size, it’s possible that using a thermoformed part for some of it may be good enough. CNCing a plastic part that big is no good, CNC’ed plastic is inherently brittle and great for models or certain types of parts, but for something handheld that probably has to withstand getting thrown in the back of a tractor or pickup that’s no good.