driftrider

You have three options the way I see it.

You can fill it with epoxy and lead shot, which will fill the space with a solid mass, but will also add weight (not necessarily a bad thing, especially if the rifle kicks hard) and shift the balance of the rifle toward the butt.

Secondly, you could go get yourself a can of the expanding foam sealant at the hardware store (the kind you use to fill gaps in a concrete wall with). Remove the styrofoam insert and fill the opening with the foam being careful not to get it on any visible portion of the stock because it's a PITA to get off once it cures. Let it cure overnight, trim the excess with a knife and you've got a permanent, hard foam filler for the stock.

Or as a third option you could just leave it be, since it isn't visible. As long as it does what it's supposed to do, doesn't cause any other problems, and modifying it offers no practical gain in aesthetics, strength, or performance, then why bother?

I'd also say that if you are thinking about having it camo-dipped, you might also consider just painting it (or having it painted). I've seen some pretty cool painted synthetic stocks, and it's my understanding that with the Krylon Fusion paint and the Krylon Web paint it's a fairly easy DIY project. There's a guy lurking around here (forget his handle) that paints stocks and has posted pics that could probably help you out if you want. You might try searching for "painted stock" here. I remember he painted a stock hot pink with black web for his daughter that actually looked pretty neat (my daughter liked it).

Mike