I just use a standard Eastpack satchel. I broke the clips which attach the straps to the bag but if you just thread the bit on the strap through the bit on the bag, you get an indestructable, cheap, washing machine friendly bag which has a pretty big capacity for its size and if you empty it, it folds up and could fit in the back pocket of your hi-vi jacket.
I just use a standard Eastpack satchel. I broke the clips which attach the straps to the bag but if you just thread the bit on the strap through the bit on the bag, you get an indestructable, cheap, washing machine friendly bag which has a pretty big capacity for its size and if you empty it, it folds up and could fit in the back pocket of your hi-vi jacket.