One simple way to suspend fruit in any container is to place it into a hop bag (nylon type)
To make the bag float, cut a small section of PVC pipe, and glue two end caps onto it and 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 have 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.
If you want to avoid all of this you can purchase Aseptically prepared fruit products
- Aseptic Products are Safe for Brewing. You can maintain control of your brewing process with aseptically packed products. Generally, these are flash heated to minimize spoilage causing microbes and yeast then immediately cool the product to maintain flavor and color.