I found that in order to take advantage of the usefulness of the excellent custom porteur rack that Alistair Spence built for my Nishiki, I needed some kind of porteur bag. After pondering making or commissioning a high-class canvas porteur bag, I decided that it would make sense to adapt a pizza bag.
The pizza bag has a few distinct advantages over a classy bag - it is cheap ($25.50 at Bag Solutions, https://www.bagsolutions.com/comersus/store/comersus_viewItem.asp?idProduct=230 ); it is easily replaceable; not so attractive to thieves, and tending to make my nice bike look less appealing too; and the synthetic materials are lightweight. It is lined with thick foam for insulation that is useful as padding as well. Here it is, when empty:
I removed the foam insulation from the top half of the bag, where it wasn't doing much for cushioning but made the bag a lot bulkier. I added four grommets so that I could bolt it to the eyelets on the rack. I cut the shoulder straps and added Fastex buckles - the straps are adjustable so that the opening can be adjusted and the straps can be pulled snug to secure loads.
Because the sides of the bag are loose, and I don't want my tools and things that I carry around to bounce out or slide around, I also made a small zippered musette bag out of lightweight cotton canvas duck. This was made from scratch, and very crudely. This bag is also useful because I can leave the bike outside and carry the musette with all my contents with me.
Here is the bag semi-full with a reasonable load of groceries:
An odd-shaped cargo of some kind:
A very large box. The bike handled really badly with the load centered way out front like that, but I made it home anyway.
A couple of rolls of 36" x 48" drawings. There were several rolls of 18" drawings inside the bag underneath the large rolls as well: