Not my tank or photo...just an image I found off the net. It's a basic metal cap with a single vent hole in the middle of it. I haven't had a chance to measure it...but it's almost the same size as a gatorade bottle opening. I took the gatorade cap and cut the threaded section in three places and it now resides on the filler hole till I can get the proper cap.
I will measure it when I am home tonight. It seems like it'd be a fairly typical "lawn and garden" type cap, I just wasn't sure the specs on it and the last thing I want to do is have to take the tank off the mower to get one fitted.