During this ongoing Hammer Jam event (which lasts for the next 12 days), the costs to build and upgrade buildings in the Home Village are halved, which include the Clan Castle. As such, you can build the Clan Castle for 5000 elixir, instead of the normal cost of 10000 elixir, and at TH2, you can hold a maximum of 8500 elixir, with the Elixir Storage upgraded to its max level (level 3). Rebuilding the Clan Castle at TH2 is, therefore, impossible unless you buy or claim a resource pack that allows your Elixir Storages to overflow, allowing you to rebuild it at that Town Hall level. Below is proof that it is currently possible during this Hammer Jam event.