Just poke your rig into the clay
I use a stick double ended that I directly embed into the clay. Double end prevents the puppet turn around the stick. Additionally I put spikes on the ground so the feet do not slip. My puppets are about 12 cm. I don't know if the method will even bigger ones.
Stikki-wax the rig onto the puppet's back. The rig can be a piece of twisted aluminum wire or something more complex. Stikki wax will attach to clay easily, especially if you heat up the end you want to connect to the puppet with a hair dryer. If you want something that will hold strongly and will not stick to your fingers the way stikki wax would, or if you need the part where the rig attaches to blend with the color of the puppet, you can use a piece of Puppet Putty of the same color.
The Animate Clay site has a section on the Claymation armatures used for the California Raisins specials on TV by Will Vinton Studios. You can learn all you need there.
to prevent the clay from turning around the rig you need to use a Fork style tip which pokes the clay in two points.
I use two wires poked into the puppet about 12mm apart, so the puppet will not rotate.
You must notice than animating with clay is not only to move the puppet every frame, you have to remodelate frecuently. The rig only keeps puppet on feet, it doesn't hold it completely in place like a normal puppet. Modelating and animating process most times move the puppet from actual position so you must place it again with "live view" almost every frame.