I usually fit my flatbed trailer with a box for glass recycling. Usually I just strap it on with rope or velcro straps. the box is good for shopping, hauling rubble/gravel/loose waste, transporting garden tools. It is also handy for carrying bikes, layed on top.
I use the plain flatbed if items wont fit in/on the box, eg 25kg cement bags, lumber, compost in bags .
How do you use yours?