I imagine a long rubber liner or roofing material might be pricey.
Another thought, in an old thread folks were getting used billboard 'tarps' on ebay for cheap. I think they are 35-50' long. They can be glued together with PVC cement per my recall of that thread. Folks used them for a liner in their pond and they were pretty tough. I would think for a narrow chute you could either fold the billboard tarp in half to make it stronger or cut in half to make it go further. A couple large billboard tarps and it should be fairly slippery for flow. Carefully putting rocks on that material should not harm it at all.
