One simple way to suspend fruit in any container is to place it into a hop bag, but use the nylon type.
To make the bag float, cut a small section of PVC pipe, and glue two end caps onto it. . . . then throw it into the bag.
One note of caution, if you are using fresh or frozen fruit be advised that the surface of the fruit will contain wild yeast. . . and you are very likely to end up with a bit of funk. Which could be a good thing. . . . depending on what you are looking for.
If you want to prevent that, you need to treat the fruit with Metabisulfite, commonly used in the wine industry . . your home brew stores likely has it.
If you are OK to risk the Funk, then you should make a solution of Citric Acid to give the bag a good rinse before exposing it to your beer - - - just don’t carry too much acid with you or it will change your beer.