Inspection/Cleanout/Test question

I am going to test my ABS plumbing with water soon. My next step is connecting the new system with the existing. I'd like to attach the 3" to the existing 3" with a cleanout but I'm not exactly sure what the best method is for that.

The connection point will be in the crawl space and running horizontally. I will want to use the cleanout to insert a test balloon. What the best way to install this? Should I use a 3" T with a screw in cleanout? If the system is filled with water, how do you release the balloon and not soak yourself?

Thanks for any constructive help.