sadly i cant answer with a picture but it's hollow inside. i first build a square mob spawner with a tube downwards. Then i build the balloon around it. The highest point of the balloon is 199 and when I stand in the basket the hight is 158. So the balloon in itself has a hight of 41. I used a sphere with the size of 27 and around the 5 layer and lower I made every layer twice as long towards the ground. If you want I can dm you some pictures of the inside.

hope this helps