Step 2: Make
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!
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!