I got a bunch of these:

https://www.ebay.com/itm/USB-Recharg...UAAOSww5hZI5bt

The price certainly is right. But quality is hit and miss. The two I got works really well, You can mount them on both seatstays below the brakes and have the on blinking. I also mounted one on the helmet using the rubber bands. They're super light. Last 3-4 hours on one charge. And cheap.