Another type of foot contact switch that might be useful. This uses the below tactile switch from Radio Shack. Bottom the switch has wires soldered on and is hot glued to the end of a 1/4" diameter wood dowel for testing. This test setup is too light to make the switch without adding some weight. The light weight issue should not be a problem for a bot which would put more force on the leg/switch. The switches are $3.99 for four at the local RS store. A rubber foot cap should work well covering the switch.
edit: checking with an old food scale it looks like it takes ~4.5 oz. of pressure to activate the tactile switch.
radioshack.com/product/index … Id=3060978
http://web.comporium.net/~shb/pix/switch3.jpg
http://web.comporium.net/~shb/pix/switch4.jpg