9 mm is 3/8” … so it would work for a shoe rack if each cubby is only big enough for one pair of shoes. And as someone else pointed out, it needs at least a partial back for stability.
On the underside of the jig, fasten a strip of ply to act as a fence, or guide. Need to do it carefully so it’s square to the slot. This is a sketch of the underside of your jig … the fence keeps the jig square to the desired slot, and the correct depth.I was getting a bit fatigued after the 20th cut on that shelving job , what with the clamping, squaring up, and adjusting the router bit length etc.