Anthony (CA Ike) here on the forum made a nice stack of those kites too. You could send him a PM and ask about his setup.
A one piece spreader is not great for a kite that size.
I would use an external aluminum ferrule at the T and load the end of each spreader with FG rod if more strength is needed.
The other option would be the molded Excel type T. They are pretty strong.
Picture of the two types below. I think the one with the set screw is a Prism part, but it's the aluminum ferrule more that the T itself you need