Test Cap Question

I am almost ready to test my system but do not know how to cap one toilet flange. I had to use a 4x3 offset flange that I purchased from ********* it did not come with a pre-installed cap. I normal knockout cap does not fit the flange and I can not get a wingnut expandable cap to fit either. How do I cap this?
Thanks

A test ball plug is what we use. Just be careful with them. Here is a link to a manufactures site. http://www.cherneind.com