[Old Man Dave]

I found heavy duty plastic bags/sacks such as the type builders sand or gardeners compost comes in to be very good. The spear passes straight through and the line indicates where. A bit of air in the top and some stones in the bottom of the bag works. If you were serious I'm sure you could make up a frame to hold the bag and either anchor it to the bottom or fix it a set distance beneath a float. Terry Maas uses a net strung on a frame in his book. Again the position of the hit is indicated by the line passing through a mesh hole in the net.

Dave