Birdshot works well. Just fill the base with superglue, dip in birdshot, and it should get enough to weigh the model down. Far cheaper than a penny or metal bits, oddly enough, too-- the amount of birdshot you use on a base is pretty negligable.
I just went and bought a jumbo pack of round split fishing weights. Pry them open, and using wire cutters seperate the two halves. Then glue them under, on top, etc.
The up side is with some sand any you add to the top base can look like smooth stones poking through the sands surface.
Also for under the base the big ones can be a little tall but some pressure, and pliers flattens them right out.
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