And it will be easy to verify, just show the gps track
(current can not be though...)
Thus you dont need a channel, you can do this anywhere, practice, and see your gps track afterwards.
A bit more difficult to keep it straight yes, but good practice (I will suck as my tacks are nothing to write about).
I am pretty sure high aspect big wings, foil kite, and the more wind the better
So the challenge could also be in how low wind you can go "narrow" upwind?
Besides the narrowest in "any" wind.
The other way could be just as challenging - how narrow can you go downwind?
In how low wind (a LOT more difficult in low wind).