Andy, Revelate Designs (in Alaska, no less) have a huge range of frame-handlebar-seat-etc bags. Their online ship website is temporarily down for re-stocking, but Bike Touring News is one of their online suppliers in the US. Here's a link to the Revelate bags at BTN:
http://store.biketouringnews.com/categories/bicycle-bags-and-panniers/revelate-designs-frame-bags.htmlThere's a "Fuel Bag" in that selection which might suit, but it's not cheap at USD 55 + shipping.
I just bought a "Tangle" frame bag that fits beneath the top tube, between the downtube & seat tube, and from that, they appear to be very high quality in materials and assembly. The mountain-bike guys in these parts praise Revelate products.
Hope this is useful--good luck.