4" heavy exhaust pipe
Scrap plumbing fittings
1" bar stock
2" bar stock
Scrap chain ling cut in half
Two 3/8" bolts about 1 1/2 long
Cut the exhaust pipe to make one 8 inch piece and one 3 inch piece. Slit the 8 inch piece in half and spread it wider. Cut four 1 1/2 inch long sections of bar stock. Center drill holes 1/2 inch from one end. Weld those to the top of your clam-shell as shown. Bend and manipulate the spread of the clam-shell and tabs until one slips over the three inch piece of pipe cut and the other slips over the tabs of the first. Drill and bolt loosely through the two tabs and 3 inch piece of pipe as shown. Take your time to shape it so that there is very little binding. Disassemble contraption and measure for internal clamping levers. Make those from bar stock and weld the cut chain loop to the end. It is important to make sure the bars are as close to center in your clam-shell as possible but yet do not interfere with each other. Longer bars will give you more leverage. How high you weld the bars above the 3 inch mounting ring determines the spread of the jaws. Experiment for your application. When everything works to your satisfaction weld the mounting plate to the back of the 3 inch pipe. It is important to position the bar so that it will not interfere with the ropes tied to the links of chain. I used a piece of 1/4 inch thick, two inch wide scrap. Weld an appropriate plumbing fitting to the center of the mounting plate. Remember to grind off the galvanizing first, it is poisonous. Trim, sharpen and curl the cutting edge of the clam-shell to match your application. If it isn't bringing material up it needs to close tighter. Tie your rope to the internal jaws and screw head onto your extensions.
You do not have to have two ropes to the top of the hole, you could tie a single rope in a Y going to each side of the clam-shell jaws. To get more leverage make your jaws shorter, say 6 inches but this will require additional curling and trimming of the cutting edge to get a proper closure. Once you have levered the rope closing the jaw you can hold the rope by pinching it to the shaft as you lift. Add rope as needed.
Like I said in the video, it didn't work as built for me but I truly feel that is a result of soil type. I hope it can be of some use to you.