My recommendation looking at it from the side and personally as an amateur without inside-scoop info (and without exact info on what you're building) is to look

My recommendation looking at it from the side and personally as an amateur without inside-scoop info (and without exact info on what you're building) is to look at sheet metal. 10mm thick metal, cut into various shapes and assembled with off the shelf parts like bolts can do a whole lot more than we give it credit. The benefit is that you could probably find local laser-cutters that could do small runs of cutting the pieces for you without the big cost of milling or injection molding. Personally, the biggest hurdle for me is "finding" the cost of some of these processes so that I could adapt my idea and material early on. All places in my local area require you to submit a formal quote and plans without giving you an indication at all of how much it'll cost (which is inside info that people who use those services often know). Oh and don't get sucked into the trap of ordering runs from Alibaba, even though you will find someone that does what you need at a cheap price there. It's a pit of time sifting through spam and incorrect info till you find a good reliable supplier, so rather deal with local providers in your area.