I have a Real Lite from RealLITE Large Flashing Bicycle Lights - Red * Gold Yellow Amber - 18 LED blinky using 4 batteries.
It comes with a seat post mount. The light weighs at least 12 oz.
They sell a rack mount bracket, but it didn't look like it would work on my rack.
My wonderful spouse fabricated a bracket to attach to my rack, using aluminum bar stock.

They really mean it with the lifetime warranty.