If, in your 3D printing, you can add a small ridge behind the Pro Micro, that should do it. You just need a firm surface behind for support when the cable is pushed in. For when the cable is pulled out, the case wall should work as support and prevent the Pro micro from moving. I have a similar setup in one of my keyboards but not a 3D printed case. To prevent the Pro Micro from shifting side to side, I just taped it down with Kapton tape. Works great.