I have always understood the Q to be incorporeal in essence. They have an existence in the continuum, which itself is also - like the Q themselves - intangible to ourselves. All of which would mean that the question of "what is their real form" is a meaningless question - like asking what colour energy is, or what shape water.
When they interact with humans, they appear as humans, and limit themselves to a perceptual form that makes sense to humans. When Janeway was taken to the continuum, the appearance of that place was perceptually appropriate for her. And if the "real" form of Q and the continuum is so imperceptible to us, then even understanding what it means to define a "real" form - what do you mean by reality, where all of what we consider real is actually rather ephemeral?