Great to see some interest in this.
For your iPad my mod the way it is made wouldn't be as comfortable do to the iPad's bigger weight.
It wold pull on the controller making it uncomfortable to hold.
But I have a solution for that.
This is what I suggest you do look at the image:
What I suggest is you use all 3 legs from the tripod. (Giving much better solid hold)
Open the DUO controller it only requires several screws taken out not that hard.
As you can see drill 3 holes about in the same place as I have them drawn. (Only just big enough for the legs of the tripod you use.)
Then glue them in whatever way you think it will fit best inside there.
(Making sure it will allow for the controller to fully close when you put the 2 plastic halves together.)
You will need any kind of: Plastic Hard Back Protective Case Cover For iPad as represented by the green box in the image.
(You wouldn't wanna glue directly to your ipad I'm sure.)
Then once you have the legs glued in the controller and it's all assembled.
Then glue the 3 legs to the Hard back cover for your iPad.
To give you an idea how: Take your hand close it. And see how your fingers are. Just like that you want the legs bent over the controller.
That's about it.
I believe it will work well and be quite comfortable to hold giving it good weight balance.
Let me know if the instructions are clear enough for you.