In principle one can make a 3d photonic crystal for microwaves using a 3d printer. I don't know if these actually have a photonic band gap, but it's worth trying. I guess this design would work for millimeter waves.Cura didn't allow me to print the 0.4mm lines, so I fooled it by changing the nozzle diameter to 0.2mm. Layer height is 0.25 mm and printing temperature is 220C. I also changed the filament diameter in the software to make thinner lines. (I could change the flow parameter instead)From left to right here I list the print parameters. (mesh number, filament diameter, print speed, )#1- 1 2.75mm 50mm/s #2- 1 3.50mm 50mm/s#3- 1 4.00mm 50mm/s#4- 1 4.00mm 25mm/s + double cooling#5- 1 4.00mm 12mm/s + d.c.#6- 2 4.00mm 25mm/s + d.c.#7- 2 4.00mm 50mm/s + d.c #8- 2 4.00mm 50mm/s + d.c + 235CApparently, the best looking one is #1, but the parallel lines tends to stick together and fill most of the space. For all, I can see through them when I look closely except #1, which transmits very little light. I can see through in the z-axis for 7 and 8 but not the other ones. It could not print the sides well, but inside of them was pretty ordered.Densities are 0.77 g/cm2 for #1, 0.53 for #2, 0.40 for #4,5 and 6, 0.29 for #7 and 8. They are quite sturdy by the way, so I think they can be used as a lightweight shock absorber.Another advantage is the permeability to liquids and gases.