Small mesh storage bag for gilet/knee warmers etc.

[PhotographerW2]

How about these bags at Alpkit?

Otherwise you can get bigger wedge bags than that. I have a Carradice one that is 6 litres which is probably a bit big but theres a number of 2-3 litre ones around.

I've ordered some of these and hope to attach one with a velcro strap. They're crazily cheap and will be useful for something if they don't work.