So, the revised plan is:
1. Use eye hooks. This means the holes you drilled would need tapping(?) So the eye bolts can be threaded in.
2. The eye bolts at the bottom would be longer to act as stoppers.
3. The eye bolts at the top would be shorter to allow the nuts to slide out.
4. An additional guide bolt halfway down the length of the channels for stability.
5. Use a stranded steel rope for better strength and handling.
What we need:
1. Eye bolts with a screw size slightly larger than the current holes.
2. A way to cut these bolts without damaging the threads.
3. Tap(?) the existing holes with threads.
4. Stranded rope if the fishing line seems problematic.
5. Additional guide bolts.