Love the snakes!
I've heard of extension tubes for macro, but I don't understand how they would help in this situation. Could you explain?
Since any amount of extension reduces the minimum focusing distance, had I used one in this case I could have stood right up to the glass. Without it, even at the minimum focusing distance of the 100-400LII I had to stand back a bit too far from the glass which allowed people to crowd in on both sides causing reflections that I had to shoot through.
And because of the closer working distance, extension tubes also provide more magnification. My 300mm has a tube on it 50%~70% of the time. It's a great combo in the field. I also use it sometimes with my 500mm when I switch from shooting birds to shooting insects. I usually have a tube in my pocket - beats carrying my 100mm macro (unless a trip is specifically going to be for macros).