As well as this, I practice Kendo and Jo techniques using a big builder's shovel. One time a few years ago, a construction company was doing some work in the garden and they left behind a shovel. I was trying to practice with a heavy Bokken but it was not heavy enough after a while so I switched to the shovel. Its very good. I do Shomen-Uchi (overhead straight cut) lots of times each day. At least a hundred strikes. Its good for the internal power.
I know one Sensei who has had the core of a Bokken drilled out and replaced with lead to give it extra weight.