This handy bag was designed with helmet  protection in mind.  But is a great size for many other uses, such as laundry bag, overnight or even tack bag.  The example in the pics is a tough 535g poly/cotton canvas ripstop, (please check in options for other available colours.)  The top is secured with a heavy duty zip and a pvc webbing strap and buckle, which also forms a handle.
 
diamensions:
Bag laid flat is 54cm across x 41cm high
square base of 27cm x 27cm
Circumference of 108cm when standing up