I don't know about filling 'em with sand...but I heard that you could use nylon plastic sheet, cut to fit to line the inside...there is also a manufacturer who offers them at a rather steep price. Myself, I use one of those nylon liners that doubles as a soft portable cooler...they have a little stiffener/insulation that works to help the bags keep shape. Of course the trick is to measure the inside and search around in sporting goods stores and camp supply places (found a set in the local thrift store)...but garage sales and others sources are possible, you have to be patient to get EXACTLY the right size.