Inverted flat spins are easier in most planes. Here is how I do it.

Start up high with an outside snap into a spin. (right rudder, left aileron, down elevator) As you are spinning down, slowly neutralise the left aileron, and possibly even add some opposite (right) aileron to flatten the spin. Now play with throttle and or aileron as needed. This is not the only way to achieve a flat spin, but one of several.

Some lightweight planes will stop and even climb with opposite aileron