The best thing I found was a thin flat piece of rubber that I used to make a gasket with. My fuel take started leaking baddly after a crash in the desert. Got me back to camp and it's still on the bike.
The next best thing as a black marker. I had snapped off my clutch lever and had just a little nub. But not enough of the lever to use it. After looking around for a while I found the marker. So pulled all of the guts out of it, mash it on the nub and tape the snot out of it. It worked well enough to get me off the trails and back into town about 60 miles away. I bought 2 levers the next day.