Drill holes underneath and fill then with fishing leads.
Woodworking
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Plaster, when dried, is going to be even lighter than the wood. Mortar might be a bit better. Better still would be to find some kind of metal weight. How big around is this? Would something like this fit into it?
Also, what kind of surface is the floor it will be used on? One of the most effective things you can do is increase friction. If it's going to be on a smooth floor, like wood or tile, it would be good to add rubber to the bottom.
It's not just weight but friction. Depending on the flooring you could add a rubber pad to the bottom of the piece to help. If you do go the cavity route, steel washers are pretty cheap and would be heavy, or as someone else suggested, some lead weights will work.
Take the box of bent nails/ screws/ nuts/ bolts/doo hickies that everyone of us has and fill it with sand.