Thread became a shopping suggestion so I guess I'll play. Wally shopping says quality is not important, just price.

Since you remove the seat post any how, Here's a tail light and seat post combination .. Built-in Bicycle Tail Light - LightSKIN

I like Dyno hub lights myself , but Attach a Light - Twofish Unlimited has a velcro strap Block for fitting a Flash light on your handlebars .