Arduino Bare bones with USB?

I am doing a project were I need an Arduino. I am thinking of doing a bare bones one. But I need it to communicate with the computer using USB. All the bare bones project I found on google didnt have it or the were dead links.

P.S: I cant use the original arduino as I am planning to use this project in an exhibition, were the guidelines say all circuits must be created by the participant(sigh) =(.

How is it "bare bones" if it has USB, that's contradictory.

Boarduino is the closest thing I can think of that you might like.

An inexpensive method to add USB serial communications for a true barebones or embedded 328p project is to use one of those Asian USB TTL serial cable modules. Works great. With a small simple mod you can even use it to upload sketches to the barebone if it has a bootloader on it.

http://www.ebay.com/itm/Serial-Converter-USB-2-0-TTL-UART-6PIN-Module-CP2102-/160587864333?pt=LH_DefaultDomain_0&hash=item2563c8590d

Are you allowed to use USB/Serial adapters like Retrolefty mentioned?
If yes, wire up a barebones board by hand, and connect Rx,Tx, Power, Gnd to the adapter module, or get a cable that has same part embedded in it and do the same.

http://www.dipmicro.com/store/SET-START2

Add an Atmega 168/328, 16 MHz xtal, 2 22pf caps, have everything you need. http://www.dipmicro.com/store/DG126-3P some of these to connect your offboard wires too.