Well ive never seen one but why not buy a couple sheets of pvc glass and just make it like a small box for them to TEMPORARLY sit in 
I've seen them! Old Charley Grimes uses them all the time on collecting trips (he's a big guy in the cichlid hobby). Basically just a skinny glass box. Charlie actually has inserts so that he can swap out the background color depending on what would make the fish look best - white, black, blue, and red if I remember correctly. You could also use specimen containers like the ones the lfs use before they bag your fish up. Put a perforated divider in that you can slide forward to get the fish to hold still, then remove after your picture is taken.