Australians come up with some pretty good ideas and AyUp lighting is a great example. Their solution to rear light visibility is to 'cap' their lights with colored extensions that give excellent 180 degree visibility. The red Saxon Caps are here http://www.ayup-lights.com/products/134/

No guarantee they'll fit all flashlights, but they'll likely fit some and give more light than most tail lights. No dimensions are given onsite.