Fantastic Jay! I know that one point of a flat pack is to be able to offer it at the lowest price possible by making a run of identical ones, keeping the number of table saw set-ups and cuts as low as possible. But since you just mentioned the frame may be offered with the choice of either an overhanging or flush top, let me ask you the following:
1- Would you consider making the same choice for the bottom, i.e. flush with the sides?
2- All that would be necessary for an inset front grill to be possible is for the horizontal middle brace to stop short of the front edge of the side panels by, say, an inch. Would you consider making a set of frames with the front brace one inch shallower? If not, it would be easy for the buyer to cut (or have cut) an inch off each brace. Then, little blocks could be glued into the four inner corners of the H-frame onto which the grill frames could be secured, for a clean finished look to the H-frame.
3- If the answer to question 2 is yes, would you consider making the grill frames themselves? Just a simple 3/4" MDF frame (picture frame style) with perhaps a strip across the middle (running horizontally, left to right) to prevent the long sides of the grill frame from bowing inward when the grill cloth is stretched tight across it. If not, it wouldn't be too difficult to put a set together by anyone desiring them, from strips of MDF.
4- Will you be offering a simple MDF box for the Rythmik plate amp?
Whether the answer to these questions is yes or no, the H-frame is very welcomed. Thanks for offering them!---Eric.