You can bungee a shoebox to the top of your rack, they are about the size and shape of a trunk bag but are significantly lighter. I suggest new balance shoe boxes because the lid is attached to the box and not a separate piece that you can lose. They can be a little flimsy though but given the weight that you can carry in a trunk bag, they will probably suffice. If you want one that will hold up in the rain, you can collect enough corrugated plastic signs on the the side of the road and build a shoebox sized box out of that stuff. Either option will provide your bike with an extra ghetto look to it.