For the platrforms I build I use extruded alumunium profiles. There are cheaper standard solutions, but i do not like the profile to profile contact solutions, or working with angle plates. Therefore i design my own connecting plates.
They ensure a very strong, symetric, and very easy to adjust and mount connection.
I make them out of black (or blue or white) Polyoxymethylene that means no surface treatment. And when mounting or adjusting the profiles you won't get scratches. They are cnc machined.