This is a custom built part for the Gilbert Engineering 37" Mini Tree

https://gilbertengineeringusa.com/products/mini-trees-rev-2?variant=39291113242726

It slides onto a ½" EMT pipe, and sits inside of the top of the tree to keep the pipe centered. There are two 4mm holes set into it to allow for 4mm bolts to keep it in place, there are also pass through gaps for wiring if you want to mount a star above.

I used PETG for it, as it will be used outside, but you could use anything you like, it is not structural, but I would suggest 4 wall loops minimum to allow for the bolts to work well.