Since the fabric choice, corner wrap technique, and method of attachment result in a hard to predict finished LxWxH, I'm thinking it makes sense to build the grills first then fine tune the H-frame opening (and grill recess depth in particular) to accommodate them.
MDF vs. plywood for grill frame? Plywood seems like it would be less fragile.
Also would be much obliged to receive links for grill fabrics that you like. Thanks!