Being a wood worker also, I work with what I know.

1" inch dowel with a spike driven ( predrill ) in the end. Bore the other to accept the leg just barely. now slit the top with a back saw so it can be compressed around the leg with a small hose clamp. Do not over tighten as you might crush the tube.

If they tend to slip off when pulling out of the ground, bore the hole a little larger and wrap the bottom of the leg with gaffer tape to provide some friction.