Hi. There seems to be a misunderstanding about how to align the three capsules. Their diaphragms need to share a single, vertical center line among all three of them. In the photos, the bottom capsule is too far toward the back of the setup.
The four parallel sets of slits in the capsule housings of the MK 4 or MK 41 are some distance behind the diaphragm. They're inlets to the "delay line" that feeds the back of the diaphragm, so that rear-incident sound will arrive on both sides of the diaphragm simultaneously, thus canceling out. Let me see if I can mark up a photograph or drawing to show you ... OK, this definitely isn't perfect, but it's closer. It should also reduce the interference caused by having the base of the "hoop" be directly in front of the rear-facing capsule: