Give players the ability to rent or win an extra builder; if they are vip they get 3 if not vip they get 2.
There can be various ways to get an extra builder, as a token in a chest, as a deal involving exchange of money or better yet duplicate the raid shield approach.
Set the length of time the extra builder is available either by number of buildings to be built or total time the builder is active.
50 gems for 1 hour/1 building
200 gems for 3 hours/ 2 buildings
If builder only has 2 hours left they can’t be used on building that takes 2 hours 1 minutes.
If the extra builder is rentable by time and the previous situation occurs those 2 hours of builder time is saved, eventually to be added to any future builder time.