PhotographyandCinema.com has just added a pair of 8" Industry Standard 15mm Extension rails (rods) for $28.99 + Free shipping (domestic USA). The rails have both Male and Female threads so that they can be assembled to virtually any length for supporting additional accessories, longer lenses, or if you need the extra room for Matte boxes. These will match up perfectly to your Gearbox GB-2 or Prime Shoulder Rig.
There are plenty of options for rails on the market, and I advise you to shop around, but for a pair of 8" 15mm rails with both Male and Female threads these prices are pretty darn good. Check it out at PhotographyandCinema.com (click here).
P&C 15mm Extension Rails with Male / Female Threads