For the first year I used 3/4 inch stone, in case I needed the box for anything else, well the only thing I needed the box for was more ballast. So I filled it with concrete to about 2 inches from the top to hold a few loose items, put pvc pipe in to hold tools.
Definitely no regrets. Sand, stone, bricks etc are not nearly as heavy as concrete. you'd have to use steel or lead to to have a removable medium and still be heavy enough. That would get expensive if you had to buy, for 25 bucks you can fill it with concrete.
Well maybe 1 regret, I should of used steel pipe, if I break a plastic pipe I'm stuck.
JB.