If you aren't too clumsy (walking off the edge), you can simply rez a box, make it 10x10x 0.25 making a platform (1 prim)and move it up into the air and stand on it. Make sure you change the name from "object" to "posing platform'" or something so you can find it again in your inventory. While the idea of a booth is private, rarely is someone up in the air at any given time. Besides, and I am sure you know, anyone can see thru the walls of your changing booth :smileytongue:
You can also find free posing stands for it. Just remember, you can only fly unassisted to about 170-180 meters, so don't put the platform at 300 meters and then not be able to reach it. Usually moving it up about 100 meters will get you out of the crowd.
When you are done, and leave, take it with you, or it will auto-return to your lost and found folder as the sandbox cleans up.