For this I cut a ~28mm hole in the side of the 1 liter bottle, near the lid. The plan was to put another 1 liter bottle over it with a slit and rotate to adjust the compliance. I didn't do that yet.
The condom was tied in a knot near the tip, and a rubber band attached. A small hole was poked in the bottom of the 1 liter and the band was pulled through.
A 3/4" PVC 90 degree pipe barb was turned in a drill to cut channels for an o-ring to seal against the outer bottle. Then I added another for the inner bottle and one on the outlet as it made the air tube a nice snap fit.
The fitting was placed in the larger bottle, the condom was wrapped over the top of the smaller one, and it was pressed on the PVC fitting inside the larger bottle. The rubber band was pulled tight to keep the condom from moving around equalize where it tends to blow up. Pulling it tighter will keep it inflating evenly. You will only use about half the length of the condom because of this.
Fill with water about half way. The hole for your band will allow the center bottle to fill as well. Don't go much above half full. That is it.
It is possible just constricting the flow of air through a valve could accomplish the same goal, but I don't know yet.
In addition to the restriction on flow, the holes in the bottle and water will give the problem some mass to have to accelerate. I think this will be closer to reality.