Yep I got that covered. I'm working on putting it together outside of the hoop first, and then going to give it a shot once all layed out. I'm doing three batteries near evenly spaced, but the electronics are going to be between two of the batteries and they will be spaced about 1-2" closer to the third battery than would be considered even.
The person this is for is a very good hooper I guess you could say. I wanted to make a 40-38" hoop, but she is looking at a 34" or less hoop. So, balance will be really important. The hardest part is working with the small hoop diameter, which becomes considerably smaller right where you connect the two pieces, using a 1/2" ID piece.
Good luck with your deal. The luck on my setup is needing to power it ever 30 inches or so, so I can basically do that while spacing the batteries out at the same time.
Hope your project goes well. There really should be a nice thread dedicated to hula hoops, this must be the 6th or 7th one on here by now.