We had a bow thruster retrofitted to a 405GL about seven years ago. It involved moving the forward fresh water tank a little aft which wasn’t a problem. In the 405 there is a good sized amidships storage section under the aft end of the forward bunk, that I use to stow a gennaker. There is also a small storage area on either side of that, and we used the starboard one to fit the bow thruster battery. It’s not a huge space, but I use an Optima battery which has a small form factor and delivers good power. We installed a conventional Sidepower SE80-185T, which has done the job. I think that’s the same model that Dufour fitted to the 405 at the factory. It might be worth checking with Dufour what they fitted to a 412GL.