During cooldown there is no risk of being under attack when you are online, of course don’t do this during a war season. Just make sure every now and then you do an attack (you need gold anyway). Only thing required are free workers. Remove some strong towers and start building a tower. Level one towers take less time than a raid. So replace some towers with new ones, start a raid and after the raid, sell your towers that are ready. Build new towers, start raiding.
Repeat this scenario and you will get many gems for a full silo plus farms. And since account sharing is possible, you could easily create lots of gems. Even by selling items you can generate the cash required for the low prices of starting to build a tower. According to me it’s too easy to get gems for ‘free’. Just make sure you put back your original towers before you leave. Do you really think High leavel kings care? They get enough gems from scrolls against their bases and why would they use it?
Most not so high kings however will definitely use it. I don’t care for trophies and lots of players also don’t. Even when we forget to put back the original towers, no problem. We make gems, more than we would ever get from normal raids.
Flare will never implement this, they don’t ruin their income.