Henry_b
Think outside, no box required.
With a pressure sprayer modified, used it a few times for the same thing, fill it cold then start the car wait until the oil reaches 40 degrees and while it’s running top it up again
You should also run it through the gears with your foot firmly on the brake that would of drawn the fluid through the various galleries in the gearbox..
Do this and recheck the level..
How much came out? and how much went in?