To grip the tree, the robot has very sharp feet at the end of its legs (where else?). The feet are made from jumbo-sized T pins, which you can get at your local fabric store. To fasten them to the legs, I made some clamps out of aluminum. I cut 8 3/8" or so lengths of aluminum, and filed a thin groove lengthwise into each of them, for the pins to fit into. Then I drilled a pair of holes into the aluminum, and corresponding holes into the ends of the legs. The clamps were then bolted down to the legs, with the pins inserted in the grooves. I left about 3/8" of an inch of the pins extending form the legs, but the length can be adjusted by loosening the bolts.