I’ve been lucky enough to see Richard teach many times at my studio in Austin. He is one of the very best teachers out there. And you cannot go wrong taking a class at Bullseye.
Deep square and rectangular molds will almost always end up with curved edges that dip in towards the center. Slower slumping will minimize this. You can probably get to straight edges if the flat piece has sides that are bowed out – but it would require testing.