If you need an allen wrench in a pinch, you can use this trick to tide you over until you find a real one. I used this to tighten down my bike seat and ride it a few miles home to a real hex set.
Step 1: Materials:
1) A Pencil (hexagonal barrel)
2) A Knife
3) No Hex Key
Step 2: Make
Cut off the end of the pencil. If you press the blade straight down into each face of the pencil (press, turn the pencil, press, etc.), the tip will snap off almost cleanly. If any splinters stick up after the break, flatten them down so the pencil can jam as far as possible into the bolt head.
Shave the sides of the pencil down to size. Keep the peelings parallel to the faces. Make your shavings pretty thin. Test to see if it fits. If it's tight, that's good.
Step 3: Big Emotional Payoff!
Insert your new allen wrench into the bolt head. Turn slowly.
If the wrench slips, it has been stripped (wood is soft), go back to step 2.
If the bolt is satisfactorily tightened, celebrate! Then do what it takes to find a real allen wrench, and get this baby tightened down for real.
It took me three rounds of cut/shave/tighten to make my bike seat rideable.
You can even make your hex wrench back into a pencil when you're done! So much versatility!