I haven't made any bags but did need a replacement piece for a purchased bag.
I bought a single vinyl tile at Home Depot and used a utility know to cut it to the size I needed. It's been in the bag for a year and still holding up. I use the bag for daily commuting so it's seen some action. Plus I have the rest of the tile if this one wears out.
I did find that some tiles were more flexible than others. Just kept trying until I found one as firm as I wanted.