I am very happy with the Thermarest Neoair XLite, but even with using their pump sack, I can't easily fill it all the way, because I end out letting a bunch of the air out while trying to close the valve. I'd like to put some kind of lightweight check valve on the end of the pump sack that will prevent this air loss.
Ideally, I'd like to find something at a local hardware store that will do the job. I've briefly researched fish tank check valves, but I think that these would be so small that they would cause too much resistance/back pressure. A quick glance at Home Depot's web site shows things that look very big and heavy, but I will do some additional "checking" to see if there is anything that looks like it can work, when I make my next trip there.
As an added note, I am thinking about getting an Exped Schnozzel pump bag and their Air Pillow UL, so I would end out making an adapter (I've seen a few DIY options online) that will connect to the Thermarest valve. I add this, just in case this inspires your idea for my fix.
I am not such a great DIYer, but I know that a lot of you are. Any ideas?