I've thought exactly the same when trying to manhandle the builders bags. They're too big to 'bearhug' and carried low they just bang on your legs. Put something dense in them (like chip) and you can only fill them about a 1/4 full before they can't be lifted. I did try one of these:
My ideal spec would be 60cm cube, waterproof, reinforced base, self opening, with handles on the bottom too.