I have used the mook jong and made my own out of PVC. I also made this freestanding, three-legged, four-armed, spring headed contraption that worked OK. All that stuff is interesting, but for my money, I am content with a thai bag (with as small diameter as possible). I also like the BOB (Body Opponent Bag), but unless I had a school, I would not buy it just for myself.
IMHO the dummies you listed aren't worth the money. If you want try something, go to a construction site and pick a large diameter (8-10 inches) PVC pipe, carve out some holes for arms, pad it, and mount it. If you need it, I can give you some ideas on connecting the arms and mounting it (you do want it to have a little movement).
I would also suggest, going to a tire store and getting some old tires (they give them away). Bolt them together and shove a long stick or two in between the tires as arms. Make sure the tires don't have any of the steel belt showing.
Another idea that I enjoy uses a double-end bag. Fix a string onto a tennis ball and hung it from the same point as the double-end bag. The string should be just long enough to let the tennis ball rest on the middle of the double-end bag. As you hit the double-end bag, the tennis ball bounces and flies around randomly. Focus on keeping your timing on the double-end bag (remember to move your feet) while slipping and/or blocking the tennis ball.
