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Design of a simple doubly clamped cantilever structure capable of localizing mechanical and optical energy at the nanoscale is presented. The optical design is based upon photonic crystal concepts in which simple nanoscale patterning of a sub-micron cross-section cantilever can result in strong optical localization to an effective optical mode volume of 4 cubic wavelengths in the material (4(lambda/n)3). By placing two identical cantilevers within the near field of each other, strong optomechanical coupling can be realized for differential motion between the cantilevers. Current designs for thin film silicon nitride cantilevers indicate that such structures can simultaneously realize an optical Q-factor greater than 106, motional mass mx ~ 5 picograms, mechanical mode frequency OmegaM ~100 MHz, and an optomechanical coupling factor (gOM equiv domega/dx = omega0/LOM) with effective length LOM ~ 1 mum.