Thru-hiker would be a good source of quality materials and one-stop shopping. If you're just looking for something to wear in the sleeping bag, and want something really light, I'd just use Thru-hikers Momentum all around. If you want to run outside for 2 AM pee runs w/o falling on your... bum or soaking through the booties, you could try either siliconized nylon or heavier nylon (Quest Outfitters is a great MYOG source, ~2oz oxford?) with some strips of Seam Grip running across the bottom as "treads." If you don't object to a more structured sole, you might even just use some thin foam glued to the bottom and siped, like they do with tires. Quantity of down depends on how high you want these. Thru-Hiker down is high quality 800+ fill and comes in 3-ounce increments. Particularly if you want the lower cut sleepie, 3 ounces should probably be plenty. Good luck!