how to make a cold of hollow objects

[gameangi]

Hi i was wondering if anyone here knows whats the right method to make a mold of a hollow part like gangliest boot tops etc. i have been trying to make a cold of the sideshow batman gangliest and boot top but i can't seem to get it right its to thin or the resin dose not fill the whole thing any ideas or techniques you can help me out with?
i have searched allover and so far i didn't find any thing helpful

thanks