Many times I find myself searching for that perfect universal vertex. Each geometry may or may not have one of those.
This thing is the universal vertex for making rectangular things.
It's fairly simple. Just a cube with holes that go all the way through on each of the axes. The default size here will allow you to screw 5/16 threaded rod through it.
It is very handy when you're trying to construct a closed space using threaded rod without a turn buckle. You can simply thread it through the end.
It's a fairly straight forward print, although depending on how your printer deals with support, a design that uses tear drops in the holes might be easier.
Perhaps another design that has even less material in contact with the raft will make it even better, and more aesthetically pleasing.
1) Print a few of these
2) Thread rods through them to form a rectangular shape