Making compost bins

As Veggie said builders bags are good for making compost bins if you think they are too big, put a post, or slat of wood or even a brush shaft at the corners tack the bag to the wood and roll it up a bit then tack again have the wood long enough to allow you to stick it into the ground, two corners would be enough for holding it if you have a problem getting wood, but better with wood at each corner and if you can't pick up a bag free I think B&Q sell them