Even easier: just buy a cheap usb stick of correct size, preferably one with a recognizable brand, open it up and replace the innards.

If you look closely there might be two issues with that. First the device that was used doesn't have the USB connector centered and second, the board sticks out the edge a little so I would guess that it would need a larger than normal case.

I think the parent might have been suggesting to develop your own board. Using something like a USB enabled Atmel chip (ATMega32u2) should be relatively simple, and there are plenty of HID libraries out there for it. If you were trying for a bulk attack, or had the funds, this would be by far the best solution.

Even a non-bulk attack can be done pretty cheaply these days. Places like seeed will do boards for a buck a piece if your design's small enough and you get a ten pack. Doing smt work is honestly more bark than bite compared to pth; a $20 toaster oven will work Well Enough for something like this.

For small run electronic prototyping it's hardly worth stuffing your own boards any more. Macrofab or circuithub can get you fully assembled boards in a few weeks in the $20-$50/unit range.