When I was a Playboy Bunny (dress style costume with clear shoulder straps) a few years back I needed boobs & had a brainstorm: Why not use balloons filled with water?
I bought a bag of multi- colored water balloons & filled them up to the point where it filled out my pushup bra, it took a few attempts so I used the brightly colored ones first. I discovered this had some weight & "bounce" to it, but wasn't very durable. My solution, after trial & error, ended up being to triple them up: I was able to do this by blowing up the first to stretch it, tucking the second one inside, blowing it up, then doing the same with the third one. I then filled the inner balloon as needed & tied it off, then I tied off the second, then third. One of the happy coincidences of this was the fact that the ends of the balloons I used took on a nipple shape.
They lasted the whole night, despite being groped by the girls I know.