OK, so I cheated a little on this one. Not exactly DIY but I think I have the ultimate lid for the Snow Peak 600 titanium mug. I might see if I can get another, lighter version made but for now, this is great. If you're a 600 owner, what are you using for a lid?
UPDATE: OK, for those of you who want one, I will be able to sell you one within a week or so. It will be an improved and lighter version with a wooden handle. If you can wait, I'll give you an update a.s.a.p. They're on their way! Also, there will be some slight design modifications including a wooden handle like the ones below. We're trying to make it lighter by using thinner aluminum as well.