Hi Christian,
here’s everything on one photo.
– The collar of the bottle is sandwiched for stability between two circles of fabric. The flap (in red) can be any type of tissue. As a valve this works the better the higher the pressure. I use it only to assist with manual inflation, so high precision was not my goal. (The dirt on the blue circles is glue residues. I could have protected the surfaces with tape, but these are from some prototype). On the inflatable seat I don’t use flaps on the “valves”.
To keep the threads of the caps tight, I use the TIZIP silicone grease for now, which to me looks like simple silicone vacuum grease.
– To secure the bottle cap I made a hole with a Swiss Army knife, secured the cord with a knot and E6000 (from China) glue inside and outside. E6000 adheres well to polypropylene, is a bit elastic and acts here more as a sealant than an adhesive. I assume many other adhesives/sealants would work as long as they stick to the polypropylene.
– as a bonus I show the garden hose adapter which connect’s Matt’s inflation blag to the bottle valve. Works really well.