One of the most important consideration is what sort of production volume you are looking at.
For example injection moulding is frequently used for high volume plastic components as cost per unit is low but setup costs are high. At the other end of the scale something like vacuum forming is good for making small volumes as the tooling can be made relatively easily but it does have limitation on the forms that you can use.
The difficulty with high volume moulding processes is that you need quite a detailed understanding of the manufacturing process to optimise a design and there may be very specific requirements for things like draft angles, fillet radii, acceptable changes of section, allowance for shrinkage etc. etc. Not to mention implications for the design of the tooling which may not be at all obvious from the end product.
Another good option for box-like shapes is to fabricate it using already available moldings or extrusions for example you may be able to find a rectangular section extrusion which can be adapted with relatively simple modifications like cutting to length, drilling holes and adding fill-in plates.
To be honest I would be quite surprised if there isn't an existing plastic box somewhere which is close enough to your requirements to avoid having to manufacture it from scratch.
The key point here is that the specific manufacturing process selected has a profound influence on the suitability of a given design.
I would suggest that the best approach is to focus on defining what properties, performance and standards the enclosure needs to achieve, such as environmental resistance, internal dimensions and any requirement for mounting points etc etc and then approach a manufacturer to discuss the best way to achieve these objectives.