Yeah, I wouldn't use them now but they were all I had to hand at the time.
Product bags that posh cycling kit comes in would be a good shout, even shopping bags from higher end retailers that are thicker than your run of the mill supermarket bags could be reused for this kind of thing.
That was just the first poly example that came up in search.
Sure bin bags work, but can be a bit delicate. I've been using a bag that some handlebars came in.