As Tony suggested earlier, maybe a cord-mount receptacle
might be the easiest route to go.
Here is a (random) e-bay 20-Amp cord-mounted receptacle,
which takes varying sizes of "SO" (rubberized) cord. . .
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170201407022
Someone had pointed out earlier that, if you're going to use
a box-mounted receptacle, that cutting out for a decora style
outlet would be the easiest, but I tend to disagree.
If you're only going to be plugging in ONE device,
then use an outlet like this. . . (Also a random e-bay item)
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=260376549688
It's much easier to cut out a single round hole. ;-)