Flip the bottle right-side up, pour the fluid in through the tube or acrylic.
Tile the bottle sideways carefully, until it's upside down. Some may dribble out, but you don't need a large amount of the indicator solution.
The amount of solution you end up with will be a function of how much fluid it will hold sitting sideways (ie, halfway to the "upside down" position). With the way you have it designed, it looks to me like this would still be plenty of indicator fluid.