For my lids I made 2, One was from Titanium which can be pricy, but the other I made from one of those dollar store pie pans and it's half the weight of the Titanium lid.
Take the pie pan and place the mug upside down on the pie pan with the pie pan in the up postion, trace around the mug, then at 12 o-clock, 3 o-clock, 6 o-clock and 9 o-clock draw about a 1/4" X 1/2" tab.
Take a good pair of sissors and cut the circle out, also cutting out around the tabs, once you have it cut out sand the edges so they're not sharp, then place the lid on your mug and bend the tabs over, viola! your done. You can also put a piece of foil tape on top to lift the lid off without burning your hands.
This lid has 6 tabs, but the Titanium only has 4, and they only cost a $1.00 for 2.
You will need to be careful not to cut yourself, (i.e... sand the edges) and also it's a little flimsy, I wouldn't want to pile a bunch of stuff on top of it, I'd probably put it in the top of my pack, the Titanium is much tuffer.