I got some 4.5 mm habotai silk from thaisilks.com and they were pretty good to deal with. Total weight of my liner is 2 1/4 ounces (60 grams!). A liner I had made out of a queen size sheet was 440 grams for comparison, and while the heavier cotton/poly would be warmer, and probably keep more sweat & dirt off my bag, that's a huge weight savings, so I'll survive. I ordered 5 yards because I'm over 6' and I wanted the liner to go above my head (the back part at least). I just laid one of my bags down on top of the silk and used a sharpie to sketch the pattern onto the silk with a 3" margin around the bag for good measure, then folded the silk in half lengthwise so the patterned piece was on the bottom, then sketched the pattern onto the top piece. To sew I just aligned the two marks the whole way, leaving about 20" on one side for a slit to help get in and out of it, and and it's 3" wider at the slit to help keep that area covered, so they're like flaps basically. It's not beautiful as a finished piece, especially with the sharpie pattern, but it's functional, and that's all I care about. Maybe I can jazz it up with some sharpie art?
A note about the 4.5mm thickness - it's fairly sheer, meaning you can see through it. That's what concerns me in terms of keeping sweat off of my bag. Dirt will definitely be taken care of. It's white as well, and I don't think it will be white for too long :P