Just get a clear whiteboard, draw the binary tree, then flip the whiteboard 180 around the vertical axis so you are now looking through the back of the whiteboard.
Honestly, it's an O(1) solution depending on choice of data structure, possibly constant-factor more efficient through your program depending on language, and if you're programming on a whiteboard it might also arguably be the idiomatic way to do it in that context.
Limited in that you can only have one tree per whiteboard, though.
"Write an FFT" is the DSP engineer interview question that's analogous to tree traversal algorithm whiteboarding. The hard part is remembering how a butterfly computation works, and you'll almost never need to implement it.