You might want to checkout www.dingy-tow.com. I am feed up with the same problem and came across this at a boat show. It is actually a rigid towing system but at anchor it will solve the problem. I haven''t got around to getting one yet, so I cannot verify qualty or price - but maybe this is what your looking for. Good luck
You can easily build one out of two 10'' PVC tubes and 30'' of line. Choose pvc tubes that are moderately stiff, thread the line through the tubes to make a V and attach a locking snap shackle at the apex of the V. Tie the legs of the V to cleats at the port and starboard quarters of your boat and snap the shackle at the apex to your dingy.
This ONLY works in calm water as it restricts the ability of the dinghy to ride waves, and the dinghy can tip in heavy chop.
For choppier conditions, tie both legs to the same cleat and now the dinghy has more freedom of motion but still will not hit the mother ship.