All of below you would need a cord to hang it. With plactic sheet you would want something to hold the upper part of the funnel open.
1. LW Plastic funnel with a tube on the bottom.
2. Cuben or plastic sheet funnel shaped from a square as below with a PEX slip fit plumbing connector on the bottom. If you do it right the oring will seal the bag to the fitting, and shove a tube into that.
3. Half of a plastic bottle and cap, drill a hole in the cap, put a PEX connector into the cap and hose it.
Shape funnel out of a square of your fabric to fit.