I'm pretty sure somebody posted a STEP file of a UNO a while back, but I can't find it (just try searching for STEP UNO on the forum and see what you get. Why is the search facility so brain dead) so I did some Googling and found this
You have to register.
I just downloaded the STEP model, it's a bit crude but should be good for getting your mechanicals right, just apply a sanity test before you trust it.
Rob