John in NM
The easiest way is to add a corner brace to the skirt behind where the legs go and use it to bolt the leg on. The bolt pulls the leg tightly into the ends of the skirt. There's a good picture Bill Houghton provided in the article linked below. I've seen it done with hanger bolts in the leg and wing nuts on the other side of the brace, and with lag bolts right into the leg. The former is better for something that will be disassembled often.
Since the legs are so big, you could consider making them hollow, out of 4/4 stock. If you added a small corner wedge at the top you could probably use a T-nut inside the leg and just bolt to it. Kind of fussy though, considering a solid plug and hanger bolt would probably be more durable. It would be good to make hollow legs though, this thing does sound heavy!