I pondered how to attach the legs for a long time. I finally settled on a method not far from Victor Franco's and Mike Senna's. I rummaged around at Home Depot and found pipes and fittings that would let me have a 3/4' diameter pipe on one side, mounted to the leg with a floor flange, and a slightly larger pipe on the other side that would fit over the 3/4" pipe inside the R2. I routed out the inside surfaces of the legs to hide the floor flanges, and did some cursing and grinding on the two pipes to get a nice fit. But eventually I got the legs to fit onto the body.
You can see the two pipes coming together inside the frame here. You can also see the big round plugs that I built as mounts that go between the legs and the upright plates in the body. I had to trim the tops and one is rotated down in the picture. Nothing is secured here--I was just checking for fit.
I'll attach the mounting plugs with screws from the inside, and I'll put a pin through a hole drilled through both pipes inside the body, as Senna and Franco suggest. That'll allow me to take the legs off later if I need to disassemble the droid for transport or something.
One problem that I haven't read about solving by anyone is how to set up this axle or the legs so that they do not rotate past the 18 degree angle that they are supposed to form from the body when R2 is in the 3 legged position. I can imagine several ways to do it.
If you're going to go for the full 2-3-2, fully automatic and motorized R2 function, then this will be solved other ways. If you're building a fixed, three legged R2, then you'll just screw the legs into the back, locked position. I am not going to have full 2-3-2 auto function--that's too hard to do with so much wood, and without a full machine shop. But I think I would like to have the option of putting him up into the upright 2 legged position manually for pictures or display, or whatever. (It does worry me how unstable he must be in that position.) So I'd like to do both. Right now I'm thinking that I'll put a pin through the frame, the mounting block and into (but not through) the shoulder of the leg from the inside. That'll hold the leg in position. Then if I want to change it, I'll pull the pin, move the legs, and put the pins into another set of holes. But I'll have to think about it.
You can also see that I've screwed the skirt onto the frame here. I'll glue a piece of white styrene onto the bottom of the frame, between the frame and skirt eventually. The underside isn't very visible, but I want to have some nice, clean finishing details there when I'm done.