There's not a lot of space under a Binnie skip wagon to add weight, so my method is an imperfect bodge.
Round balls of lead from an old scuba diving weight pack are dipped in Poundland 2-part Epoxy glue and placed under the central spine of the chassis.
A line of them raises the weight from 50 to 65g - not a massive increase, but it feels useful and in the hand, the model certainly seems more solid. They roll better too, less rattly then the pure plastic version.
Sadly, you can see the bottoms of the balls, but I'm not going to worry as they will be less obvious once on the track and the dirty grey merges into the background quite well. To be honest, since they stay on the rail, I'm happy.