Hunt around and find a supplier that does stuff for Fiberglass. Check the phone book or search online in your area.
Phone them and ask to buy a container of Micro-balloons. Take an ice cream bucket or similar size container to their shop and have them fill it. It will feel like it's empty.
For perspective, a pound (half-kilo) of Micro-balloons, is a bag about twice the size of a modern plastic grocery bag. Or a bit larger.
These are also available at many Model Airplane Hobby Shops, used for the same purpose you will use these for. Making a lightweight, strong filler material to fill gaps and form surfaces. But the Glass shop will sell you a pound bag for about the same price that the hobby shop will sell you a small plastic tub full for. Think small size chip dip tub.
Anyways, what they are is glass bubbles, very thin walled and light weight. You mix them with epoxy or fiberglass resin, as the case may be, and keep mixing until you cannot get more to integrate in, and then mix until those do mix in. It will make a dry-ish white paste that weighs almost nothing, but is as rigid as if you filled the area with the epoxy or resin. Try to get it as thick a mix as possible (lighter).
Way cheaper than arrow shafts unless you have a free source for those, and lighter too.
Now, the warnings. It's Glass. Try not to breathe in more of it than you have to, wear a dust mask. Same if you end up sanding the resultant mass. Not like epoxy dust or fiberglass resin is good for you either.
Spray foam. Get the Low Expansion stuff. Have had friends fill up plugs that they were using for mold masters, be woke up in the night by the sound og the plug exploding from the built up pressure. Said it sounded a lot like a gunshot. Made a mess on their bench too.
