If you do not have an extra piece of grip gear,consider using a step stool,stepladder,chair,or a box of one type or another. A reflector can be taped,propped, or wired, or strung, or roped as well, as long as its position is helpful
Yes, I remembered I saw someone on the net connect 2 of those clips together (with a nut and bolt) and have one clamp on a post and the other end on the reflector!
Similar to @Dao, I use a Flashpoint ball head reflector holder that mounts on top of any light stand. This one is a little more robust, in that you can angle it in any direction rather than just vertically. I use it for fill for most portrait shoots.