The Bridi T40, T60, T20 are some of the best planes there are for flat spins. I also had a Sureflite clipped Cub that would go perfectly flat. I even had a MEN trainer fall out of a stall turn into a perfectly flat spin. I couldn't recover. But the rate of descent was so low it was not damaged.

Just do them with a full tank of fuel or at least more than a half tank. The fuel slings to the front of the tank.