I’ve been using a USB rechargeable version of the smart r2 (as well as the normal one) for ages. Didn’t know it was called the rl321 as well, helpful as that version is cheaper!
I just picked up a back up rear or front marker light from decathlon today. Either for backup on the bike or for visibility when out for night walks near a road. £8, USB rechargeable, decent brightness, red and white led in constant or flashing mode. Plus a couple of different mounting options. It called a 500 cl. Not used it in anger yet, it’s charging at the moment, but seems decent so far. Good size too