You wish to build your own USB condom because you want to be absolutely, 100% sure it's built without any kind of backdoor and you don't trust anyone else to do this for you?
I wish to build - or I wish someone would build - a USB condom that passes through USB stick drives, by reading files on one side, and emulating a filesystem on the other.
It's not because I want to be 100% there is no backdoor. Rather I want a minimum of safety when I have to access a USB stick given to me by a stranger.
(Speaking of building your own, simple power-only USB condoms - like you linked to - are actually pretty easy to make. They have been used to teach (SMD) soldering at CCC events, hackerspaces and the like.)
Because otherwise you can just get one at e.g. http://int3.cc/products/usbcondoms
I have had one for years (a no-name one, not the one I linked)