The Neo Air has a short internal tube made of a thin plastic film, which is bonded to the rim of the outer tube opening that is part of the shell. The internal tube acts as a valve (similar to the type of valve on some disposable inflatable pillows), whereby the internal pressure causes the valve to close up, thus providing an airtight seal. Maybe this is the way to go – seal a thin TPU tube (2 rectangular strips bonded along the two longer sides) around the rim of the roll-top opening and let this tube hang inside the packraft acting as a valve. Obviously this works for the Thermorest, and I have had success with this type of valve on an air bladder. Definitely worth giving a go. I’ll report back once I’ve had a go.