Agree with Venkat here. You are looking to add weight only. Make sure the sand is sieved and does not contain particles or other stuff. When I set up some bookshelves, I have lined it with construction quality plastic wrapping bag into it. My friend got a tailor to stitch it so it fit in the wood stand neatly before filling in. Make sure you ahve 2 - 3 layers of lining as you dont want to ruin the stands by any chance. If its metal, make sure the one layer of plastic is the toughest one you can find.