Many people think that being second to act in game creation for clan wars gives your clan an advantage and there's a lot of merit to that. Knowing what maps and settings your opponent chooses gives you insight into their strengths and may change what you were planning to use for your home maps.
One very easy way to avoid this is to pick a 3rd party (likely a mod that is in neither clan and could be stated in the initial terms of the war) that each clan pms their game list to that is due 1 day prior to the date that the games must be created by. Once the 3rd party gets both lists, they post in the war thread that they have them and the games can now be created. (eta: if they don't announce it, it is understood that games can be created the day after the due date for the pms). Once both sets have been created, they copy and paste the pms into a post in the war thread and it is up to the clans to confirm their opponent stuck to their original list. This way results in very little additional work for the mods. Basically, all they have to do is say they got the lists, and then post the list after the games have been created. That is literally less than 2 minutes of work for the mod.
Failure to stick to your original list could result in forfeiture of non-conforming games. Perhaps a "yellow card" for messing up a setting (upgraded to forfeiture of a game if the mistake is repeated) and immediate forfeiture of games where you changed the map.
This is a nearly bullet-proof way of ending what has long been a source of frustration for many.
Unlike many suggestions, it requires no new coding and relies on the existing infrastructure