I picked this tip up off one of the US web sites. I turned a piece of plastic that fitted inside the powder dispenser hopper, then I placed a big nut on top, but anything heavy would do. I guess you could cut a piece of heavy card it would be ok.
The weight on top of the powder does seem to increase the accuracy of the powder dispenser and there is no need to keep topping up your hopper. I was consistently getting 0.1gm or less variation with the loads I was throwing.