After spending more than 10 hours with the phones, I'm glad to say that the phones are stable with 0.5 mm spacers. And they certainly sound very good with my KGSSHV.
Here are the details of my build.
1. The active area is 120 mm x 76 mm.
2. The perforated holes are 2 mm with the exception of the large 6 mm holes in the middle
3. The spacer thickness is 0.5 mm.
4. The diaphragm is 3 micron.