I started using wooden legs with a bolt in the bottom, but this is tricky to mount and a lot of work. My latest modules all have legs from CV-pipe of 22 mm in diameter. I bought special plastic parts to fit into these pipes, in which special feet are going. The price of the plastic part and the feet are less than 8 euro together. So together with the pipe it is not a lot.
I have made to connection at the module with two triangles, which are mounted in the corners with holes of 22 mm in them to feed the pipes through. On the to triangle is a stop for the end of the pipe. In the side of the triangle a bolt with eye is mounted to prevent the pipe from falling out.
I can even make the hight adujstable with big jumps, by putting a small piece of pipe on top of the other, with a soldered connection. The normal hight of my modules is 120 cm above the ground.
Met vriendelijke groeten,