Okay, so project done. I'm not exactly handy so bear that in mind. Some things to consider:
1. I don't have a proper shop. This was done on my office floor (frame cut-out and sanding) and at my dining room table (gluing frame, applying cloth)
2. I wasn't looking for perfection. The hunt for factory grills goes on. This was a function over form project.
3. As I was warned, the grill cloth was stiff and hard to work with. Not exactly a terrible thing.
Upon completion, here's what I found along the way:
1. Frame cutout is easy. No matter how much you stretch the Parts Express cloth stuff, it's never going to be flush to the face of the frame, so there was no need for the cross-member. In order for it to go between the midrange and woofer drivers, it was going to be 75% of the way to the top of the grill anyway, so it wouldn't have prevented bowing (which didn't end up happening anyway.)
2. Hot glue was perfect. Used the higher temperature of the dual-temp gun with dual-temp glue sticks. Some blistered fingers, but no worse than a night of playing hard on a Telecaster.
3. Hacking off the nubs from the corners of the frame was a good idea. I didn't want to drill these cabs. I wish Parts Express had a kit without the nubs, but the plastic sawed easily so it was no big. Industrial velcro holds nicely. When I get the factory grills, a touch of goo-gone will get the adhesive off the cabinets.
Overall, I'd give the whole thing a C+. I did the whole thing by eye, based on a visible line in the cab from where the light used to hit it with the old grill on it (bit of sun-bleaching on the lower cabinet.)
Here it is:
