There are several options you have:
(1) Surface Mount - fastest, easiest, protrudes into space, less appealing, but is funtional. Requies a single hole through wall, etc for wire & clamp;
(2) Recessed Mount - takes longer, need jig-saw for the cut-out, does not protrude above surface, most appealing.
I would use Plastic. There are several types of plastic models available. Some which have cantalever/side-arms which when slipped into the mounting hole, snap side-ways to secure the duplex gang box securely to the wall; some which you can screw directly (generally four screws - x2 top and x2 bottom).
I think you just need to dig a little deeper at Lowe's or Home Depot, or Ace or go to an electircal supply store. Explain what you're trying to do and they'll hook you up. I'll try to dig up a couple links for you.
Here is one link for a single-gang work box with the compression side-arms I was discussing - very nice, indeed:
http://www.lowes.com/lowes/lkn?action=productDetail&productId=201705-223-BH118R&lpage=none
and another nice one:
http://www.lowes.com/lowes/lkn?action=productDetail&productId=37117-223-B120R&lpage=none
Here is the link for all the gang boxes at Lowe's. In case the link doesn't work, go to Lowe's and search for the two words: gang box
http://www.lowes.com/lowes/lkn?acti...544&Ne=4294967294&Ntk=i_products&Ntt=gang+box
Here is one link for a surface-mount:
http://www.lowes.com/lowes/lkn?action=productDetail&productId=130197-223-HB1DP&lpage=none
Good Luck and don't forget to use the correct wiring sizes and ensure correct polarity (e.g. Positive to positve & negative to negative on the duplex outlet - it is labeled on the back of the duplex outlet as "hot wire"). This will ensure you maintain the correct polarity of your AC system.
Norm