Ship get some of the floating styrofoam noodles they use at swimming pools - you can usually find them at Walmart and dollar stores a single one will make A LOT -

Swimming Pool Noodle Foam from Sears.com

Get a hollow core drill bit and make a bunch of them up in small sizes to fit in the tubes - as you can see in the chart they have them down to 1mm. Longer the piece the better they should float.
Diamond Hollow Core Drills | Jewelry Making Supplies - Drills / Drill Bits