We plunge cut 4" square holes with the chainsaw about 10 ft back from the edge of the hole, one for each diver including the safety diver. Then take a 2 ft length of 2"x4", one for each square hole, securely (non-slip knot) tie the 3 tether ropes one to the middle of the 2"x4"s. Push one 2x4 down each plunge hole. The wood will float up to the undersurface of the ice, securing the ends of the tether. Getting the wood out at the end of the dive just requires patience and a screwdriver to jab the wood, push it along until the end can be fished out of the square hole, and the tether untied. Cheap, secure and cheerful.