Plastic connectors for Aluminium* square tube. Or printable square tube.
Similar to those used to assemble furniture from the homeware store. Qubelok being one type fairlite.com.au/qubelok.html
There is a zip of the STLs down the bottom of the list.
One-inch or 25.4mm profile as writen, but you can change size via OpenSCAD.
The SCAD has callable modules to model the connectors and the tube profile and a print module to split a part in half and rotate for 3D printing.
Work in progress, probably needs refining.
I'm also working on a hollow version so cables etc can pass through the joins. But no promises.
* aluminum if you don't speak true English ;^}