I have a 2009 Trek Fuel EX-9. Like most modern bikes, it has a replaceable sacrificial derailleur hangar that snaps off when you crash on the rear derailleur or catch a branch. This is great, and I always carried a replacement hangar with me, but the first time I broke one on the trail I quickly realized that we did not have the tools necessary to remove the hangar, as it is held on by part of the suspension that requires a 22mm wrench and a reasonable amount of force.
This is the solution I came up with and built with a hacksaw, dremel, and hand drill in my apartment - I used a piece of stainless steel, but aluminum would be easier to work and should be plenty strong. Mine weighs 33 grams and will also open beers.