Heavy duty large trash bag for raincoat or bivy. More durable and wearable than a cheap space blanket. Can get them in orange or bright blue or red.
Certainly not a bad idea. Wal-Mart sells some $1.00 ponchos that I always keep on hand. They're not all that sturdy, basically a single use item, but good enough to keep you dry for a short outing. A heavy garbage bag might be a little more versatile.
I've purchased three or four of those 10 packs, both in Black and Clear. IMOHO they're very good lights. The day I got them I accidentally left one in my pants pockets and ran thru the washer & dryer- and it still worked! I love that they have not just a squeeze momentary switch but also a sliding switch for constant on. The ones with a momentary switch only are almost completely useless to me.
I have ten or twelve of those tiny ones, as well as at least a dozen of them in different sizes. Absolutely fantastic 'steels! You can get a hacksaw blade for a couple bucks and cut it into small pieces with a tin snips or heavy shears and get many strikers (I have a 1" x 42" Kalamazoo grinder that I use to clean up the cut up blades but it's not strictly necessary).