I have an internal hydration system in mine. It consists of a 1.8L Platypus Hoser reservoir and an Osprey quick disconnect kit. I can easily fit 30oz in it with the DI2 battery and the tool pouch.
There’s a bit of a procedure that needs to be followed to use this. You’ve got to put the reservoir in the storage area EMPTY first. Trying to shove even a partially full reservoir inside will be very difficult, and you’d maybe get 12oz in. The quick disconnect goes up near the bite valve, and this is how you fill it. Make sure to put the tool pouch and anything else you want in the storage box before you fill it. You’ll need a second reservoir, fill this one up with the water/drink mix you want and use the disconnect kit to connect this reservoir to the one in the frame. The water will flow into the internal reservoir until it is full. I hope that made sense, let me know if you need more clarification.
This system is not refillable on the fly, and I intend to only use it for my concentrated nutrition. Also, once the reservoir is full, you no longer have access to the DI2 battery or junction box (if you put it down there), so keep that in mind.