When started assembling my quadcopter I hated the thought of having to drill holes in the HobbyKing frame kits to get the APM2 mounted.  

So I took some measurements, made these adapters in Google SketchUp and submitting them to Shapeways:

Here are some more pictures, please forgive my lack of proper hardware, and bad camera, but you can see how they work.

 If you think this is something you want you can have shapeways 3d print them here.  For the White-Strong-Flexible they are $2.16 each.  I'm sure having them made from plastic and not 3d printed would be way cheaper, but I have no access to manufacturing like that...

http://www.shapeways.com/model/604513/x550-to-apm2-adapter-v0-1.html