ETC R10 is a perfect choice for unbelivable price. The durable waterproof light is surprisingly bright and long lasting. 
Key Features:
 
Light source: Cob red LED rear light
Lumious Flux: 10 lumen
Steady-1/2 steady-flash-off
Battery: CR32032x2 (Included)
Waterproof: IPX3
Bracket size: 22.2-31.8mm
Installed vertical or horizontal
Product Size: 61 x 17 x 32mm