In case the commercial feed-through made from flat copper braid proves too flimsy, I will experiment with a flat copper strip. Here are my thoughts that might be a bit overengineered.
It involves two 3d-printed case halves for isolation with a ziptie slot for strain relief, a lasercut copper strip and a wago clamp to connect everything electrically.
I can already think of a version that omits the lasercut copper strip and works without the expensive lasercut part.
I probably also need drainage holes for outdoor use.