I've been using this vinyl/LED reflective safety triangle. It runs on AAA batteries and lasts for quite a while.

My current setup is a cargo trailer, usually with two storage totes. I tied the triangle to the rails of the trailer and then use a bungee cord to hold up the top part. If I'm hauling particularly large items (without the storage totes), I can also bungee the triangle up over the items. Action shot: