I work in robotics and this is very common for prototyping and also common for grad student projects. Here in Japan, Misumi will machine things like that to length and add whatever features you need, or even configure an entire assembly (eg a 6 sided box) and get all the parts delivered. I don’t recall the name of similar sites in other countries but they exist.
Other easy things to get a good result is use nuts or inserts instead of threaded holes when you can, and try to keep printed plastic parts in compression, not tension.
I think not enough people use sheet metal. Doing as you described plus sheet metal to put some material on the outside of the cross section not only looks professional, it adds a lot of stiffness. And for aluminum sheet 0.5mm or 1.0mm thick you can literally cut it with a knife and straight edge, and use a couple of pieces of wood or even books to make surprisingly nice bends. For thicker or more complex parts, it’s pretty easy/cheap to make or buy a decent brake, and also pretty cheap to get parts made from sites like I described above.
Other easy things to get a good result is use nuts or inserts instead of threaded holes when you can, and try to keep printed plastic parts in compression, not tension.
I think not enough people use sheet metal. Doing as you described plus sheet metal to put some material on the outside of the cross section not only looks professional, it adds a lot of stiffness. And for aluminum sheet 0.5mm or 1.0mm thick you can literally cut it with a knife and straight edge, and use a couple of pieces of wood or even books to make surprisingly nice bends. For thicker or more complex parts, it’s pretty easy/cheap to make or buy a decent brake, and also pretty cheap to get parts made from sites like I described above.