A can of Great Stuff Foam, VINYL gloves (like the green ones they sell at HD), and rubber bands
Step 2: Start filling
Start with the fingers then fill the palm part, and rubber band shut
Step 3: Now skin them
Let them cure a day or two and then skin the glove off, your see why i recommended the vinyl glove, the nitrile glove I used stuck to the foam, I did a couple dozen a few years ago with vinyl gloves and none stuck.
Once you skin it you have a fat hand you can carve or just hit with glow paint, here's a couple sitting in a pile of horse sh*t.