This project came about because I wanted some shelf mounted lights in my projector room.
To facilitate this, I bought some Ikea Dioders, and while these work great, the light bleeds out from the shelves on to the projector screen which wasn't the effect I was looking for.
To work around this, I simply cut an A4 sheet of paper in half, folded the halves down the middle and balanced them on the shelves above, thus blocking the light a little.
I had long since considered using a 3D printer to replace these, but having 12 of these strips at 30cm long, I considered this a massive waste of filament.
A friend of mine suggested using aluminium U channel rods which you can get very easily at most hardware stores (I got mine in Brico here in Antwerp). They're sold as 1m lengths which I cut into 3 x 33cm sections (this can be done with a Dremel and a circular saw attachment or similar).
You can get these in various sizes, but I went for 10x13x10x1.5 mm cuts, but others are available - the model is parametric, so you can change it suit the material you have using:
Note that you should add 0.2mm tolerance to the width/height of the rods.
To put the lights in place, I used blutack - I still have to complete it by drilling cable holes through the back of the shelves, but all in all, very happy with the results so far, and I have many other ideas for use of them (some related to lights, some not...).