Make Prep day 23.5 hous instead of 24.
Ive seen some suggestions to make prep day 12 hours, or even less, and I agree those ideas have problems. Why not make it 23.5 hours. The advantage of this is clans who war everyday can still start war at the same time everyday, instead of a slow drift away from that clans ideal starting time.
If war one starts at 7pm, they can start a new war but a match may not be found until 7:20, etc. Then eventually they might take a day off to get back to the 7pm starting time. If prep days was 23.5 hours, it would be easier for each clan to keep to the schedule they like.