yes the main one has to be UNISTRUT, its a simple chop and bolt together metal framing system that also has so many fitting bends and other premade parts, it is the dogs danglies
its sooooooooooo simple even an electrians apprentice and a plumbers apprentice, and very very occasionaly a ducters aprentice can use it with out making to many mistakes
this is my bicycle home made trailer , i have used two bend brackets and a cross of unistrut to make the main frame,
dont worry about the holes you see they are there to help and can be set up at any angle or position its not just via the holes,
the very old stuff used to be called dexian but they went bust but people still make dexian like stuff