Not OP, but I do a 36-hour fast once a week. I drink black coffee and water, but I mix one very small salt packet (probably no more than 1/2 tsp) in with one 8-oz glass of water around noon for "lunch". For the most part, that does well enough to stave off the next-day side effects.

I keep tiny containers around to pick some up with the touch of a wet finger a few times a day. Helps me experiment with how much keeps the keto flu at bay. Also frequent MCT oil from a squeeze bottle.