I am increasingly convinced that one powerful way to develop a voice that sounds the same from top to bottom, and from bottom to top, is to really slide and feel connection between pitches. And I’m a huge fan of not singing thirds or steps all the time.
Try this exercise on an open vowel, starting the first pitch with an M or an N so that the sound is firmly in the mask. Move up by half-steps for three sets, then down one half-step and start up again, so that we avoid the inexorable tension of moving higher, and so that we cover some of the same pitches over and over again. Listen and feel diligently to be certain that no pitch bumps or crunches. The aim here is total uniformity and unification from top to bottom.