Same way you did it with the chamber. I have a round disc cut to size for that very reason, you could probably make a circle one out of the cd case, maybe use a soldering iron as a cutter (fumes!).

For increased accuracy you COULD use a variety of syringe sizes but the variables can be a bit of a concern.

Just extract the fluid with the syringe once you've measured the dish.

H75 was 31
GX was 29
H89 was 29

All approximate of course.

Have round port measurements somewhere.