Easy. You get a bit of heavy foil, say from a pie dish or one of those instant barbeques, you draw round the top of your crusader, then cut it out leaving a good 5-10mm spare all the way around. Then you roll the edges over twice to hide the sharp edges, so that you've still got a few mill spare all around, lay it on top of the mug, and fold the edges over - you'll need to crimp 'em a bit to get round the curves.
Hey presto! You now have a crusader mug lid. And it'll fold in half for easy storage.
While you're at it, use the same kind of foil to make a windsheild. It makes a big difference! Again, be very careful of the edges.