Spare emergency pannier that can be stuffed into another bag?

Bit specific this one...

I've got a big shopping pannier that's easy to take off and carry round. Sometimes I buy a bit too much for it (specifically, sometimes I buy too much for the pannier, the rack and the front rack). On those days it would be handy to have two panniers. But two panniers are a right hassle to carry round, and I'm not happy leaving anything on the bike. Plus on most days one is fine.

So what would be ideal is a pannier version of those nylon foldable shopping bags you can keep in your handbag. Has anyone come across something like that, or is it DIY time?

I don't know of anything but it's a cool idea. A panner is just a bag with a stiff backing board plus mounting hardware. The collapsible bag part is easy, the problem might be getting the stiff backing board to fold up. A DIY option might be to make a concertina folding back board (strips of plastic with duct-tape living hinges?). The mounting hardware could bolt through the plastic strips. When unfolded, the backing board could slip into a sleeve on one side of the bag component.

Once upon a time a client came to me wanting to develop a fold-out tray that attached permanently on the side of your rack, so you could fold it out and strap your handbag/shopping bag to it if needed.

Another option could be a bikepacking bag, my old Bikepack.pl seatpack isn't as rigid as some others (although I see they have changed the design since I bought it) and crumples up to the size of a small melon. It's maybe 12 litres, so pretty small compared to a pannier, though.

I seem to remember @edscoble talking about lightweight panniers a while back.

Cheapo dual sided panniers would do the job quite well. They have no hard backing or mounting hardware, they are simply swung over the top of your rack and have some ties/velcro to hold it in place. They arent made of the most flexible fabric but can definitely be folded up.

Aside from this how about a drybag lashed onto your rack?

Or simply tying some nylon shopping bags like you are after, onto your rack? They could easily be adapted so they dont come off and dont tear.

I've been looking at these, Altura ones, it says in the blurb they roll up. Can't tell if there any good though from just the online info, anyone seen them in the flesh?

The Altura ones do look like what I want. I'll try and find a shop that stocks them. Thanks babapapa

Otherwise dfp and lae have given me some good ideas for diy

Alpkit or similar drybag/bag and some bungee cables.

Those do look really neat :)

Those Altura ones are a new product for this year, they roll up very small. They're waterproof too from what I remember, roll top closure. Build quality seemed good, like and Altura ARC bag without the hard plastic insert.

I did, the best one is the Carradice Super C which have a very low weight, waterproof and great for commuting.

The Arkel one is 450g for a pair which is extremely low.

http://www.arkel-od.com/en/dry-lites.html