Depends how big the space is, doesn't it? I used dropper mounts for my cages so that space was taken up with bottles allowing me to fit a Revelate Tangle top tube bag.
Or you could just duct tape a spare tube or similar into that space?
I am using a tangle as well, but no need to drop the bottle cage (although I thought about it for a while) as I am fine with 2x600 bidons. Let's say the space there is approx 10x10cm, I could fit more than just a spare tube.
Top tube bag? Revelate's seatpost bag - Jerrycan?
Depends how big the space is, doesn't it? I used dropper mounts for my cages so that space was taken up with bottles allowing me to fit a Revelate Tangle top tube bag.
Or you could just duct tape a spare tube or similar into that space?
from: http://bikepacker.com/beginners-guide-to-packing-bikepacking-bags/