Get a TechShop membership, take the classes in injection moulding, CNC I, CNC II, and CNC mill. You get free access to Autodesk Moldflow, an injection moulding simulator. Learn to use it. Design and mill your mold. Make some parts using TechShop's injection molding machine. Make a new mold if you're not getting what you want.

Completely straight sides make injection moulding harder. The tooling for straight sides has to come apart to release the finished part. But a shape that's smaller at the bottom can be made in a far simpler mould. This is why cheap items such as plastic glasses are tapered so much. Look at plastic parts around you, and notice that most of them are shaped to come easily out of a simple mould.

Consider making the outer shell by vacuum-forming. That's cheap and easy, and the tooling can be made of wood. TechShop has that capability, too. It works better for shapes that aren't as sharp-cornered as a cube, so consider modifying the shape to have rounded corners.

Both of these processes will produce a much smoother finish and a stronger part than 3D printing.

Welcome to manufacturing.

My shop recently did some work fixing some injection molds that a customer bought from a Chinese manufacturer for about that price. I was shocked by how cheap they were. Of course we were being paid to fix them because they weren't quite right, but still, $5k. Wow. If you want quality 3D printed parts, that amount of money burns up very quickly.