Looking at the model, I would say you might want to do a 3 part mold. First, get some clean clay (that is a name brand) to build up the negative. Cover everything in clay except for one side. Make sure to expose the front and back leg half way. Build your mold box to fit arround the clay frame (as outlined in Glassman6 instructions). Make your keys and pour the RTV. Flip the mold over after it cures and remove the clay down half way, exposing the same mirror image of the side you just made (legs half way exposed). This should leave the center piece between the legs filled with clay. Make your keys and pour your RTV. Now remove the rest of the clay between the legs and make sure to use mold release on the two halves of the RTV exposed so they won't stick to the third piece. Pour the RTV and let it sit. Now when you pull the mold apart, you should have three pieces. By now if you have not done so during the making of the mold, make sure to put in you sprew and you air vents. You can cut these out using an exacto blade. Now reassemble your mold, tape or rubber band it together and pour your resin or plaster or what ever your copy material you want.
I hope this helps.