I tried out the plastic bottle idea and had mostly positive results.
I typically avoid purchasing brands owned by the Resnicks, but I made an exception and bought a 1L bottle of Fiji Water because of it's larger displacement and squarish shape. I peeled the labels of the bottle and they left a bit of sticky residue, but I didn't mind because I figured the stickiness would help prevent the bottle from sliding around. I attached it to our
Mavic Air 2 with three wide rubber bands and took off. Once I could see that the bottle didn't seem to have a significant impact on the performance or stability of the drone I started flying around the lake I was at. It seemed to go pretty well, but I have to say that the confidence I gained by knowing that the drone wasn't likely to sink was offset by my concern that the bottle would shift around and interfere with one of the rotors. At one point the bottle cap appeared on the screen and my heart almost stopped, but I quickly realized that the drone was just tilting as I flew forward and I could see the bottle cap because the camera stayed level. I was about halfway through the third battery when I noticed a loud buzzing sound coming from the drone. There was some interference on the video feed, but the drone was still handling just fine so I didn't freak out and carefully flew the drone back to the boat.... Sure enough the water bottle had shifted just enough that it was contacting on of the rotors and the spinning blades had already cut a 3/4" gash into the bottle. Amazingly, there was no visible damage to the blades which were brand new. It didn't even rub the silver paint off the leading edge of the blade tips. I removed the bottle and took off again with no trouble, the interference on the video feed disappeared and I'm sure it had been caused by the vibration from the blades hitting the bottle.
Now that DJI has added flyaway coverage to the refresh plan, I don't think I'll try the bottle idea again until our policy expires. If I do try the bottle again, I will look for a more narrow bottle that gives the rotors more clearance. I may also try double stick foam tape or self adhesive Velcro in addition to the rubber bands to ensure the bottle doesn't shift around during flight.
I like the bottle approach because of the obvious cost savings, plus the satisfaction you get from a DIY solution, and the ease with which it can be removed. I'm also pretty forgetful and if I don't remember to bring the float on the trip, I can stop at any convenience store and get the materials I need to replacement. I hope someone will find this info helpful. Credit to ddbros for the original clever idea.