I've been using foil for the lid on my SP 600 mug, it works just fine, a bit flimsy and I have to reshape it every time I use it and occasionally make a new one. Recently I switched to a piece of foil left over from a wind screen, better but not a neat and clean as I wanted. On a recent trip, my friend gave me this lid idea and I wanted to pass it along to everyone using a SP 600 mug.
1- Take a can of Bumble Bee Sensations tuna
2- Remove lid with a "safety" can opener preserving the clean edge
3- Flip label side down and you had a 4 gram lid with a tab for lifting the lid off. No need to punch a hole in the lid, steam will escape around the edge. A little heavier than foil, the same weight as my windscreen foil version, but I like it and it takes less than one minute to make.