I've never tried cutting the slots myself, but thought about using my Dremel table saw. A better quality table saw would probably work much better, but I don't have one.
Select a fine tooth blade the width of the rib slots, and adjust the blade height for the rib slot depth you want. A fine Dado blade might give you a more precise slot width.
Clamp a piece of music wire exactly parallel to the blade, and use that as a guide for cutting the slots. It should be placed at exactly one rib space from the blade. Slide in the table miter guide, and adjust for a true 90 angle.
Lay the LE/TE piece flat against the miter guide, with one end just touching the music wire guide at one end. Now slide the miter guide and LE/TE stock slowly over the table saw blade to cut the first slot.
For each subsequent slot, index the previous slot onto the music wire guide, and cut the next slot. If done correctly, you should have perfect rib spacing between slots, and from the outside slot to the end of the LE/TE piece.
Then let me know if it works, so I can try it.

Bill