I've used many materials for weights; plasticine is pretty good except on really hot days when it can slide off and leave a mess on the benches. Now I use small plastic craft pegs with a lead weight attached. I have a variety of weights for different sized branches and they can be reused each season for many years.
Although this is the smallest Willow I've tried, I have one that is only slightly bigger and it's been going for 10 years without any problems. However with such a fast growing species there is always a compromise - in this case branches get too thick and taper is lost quite quickly and the whole tree needs a major reconstruction about every 5 years. Never a dull moment!Can you contain that energy long term ?
Matt.