I can see how the carbon fibre case could be quite heavy and overbuilt in some cases.
I've been looking everywhere for a front bag that isn't rediculously heavy. I've read that Jan thinks they are light, but those bags definitely are not. He has pretty archaic views when it comes to handlebar bag material. Those fabrics, sure they are durable, but they start off heavy, and in order to become waterproof they have to soak in a bit of water to swell their fibres. Yup, this only makes them heavier! I've no idea the proportion of water absorption, but I have no doubt its a significant percentage of the original fabric weight. I'd like to see someone with one do a test to see how much their bag accumulates.
I've ordered some fabrics to make my own. As I haven't done it before, I didn't go crazy, but I'm sure the finished bag will save at least one or two hundred grams over the Gilles Berthoud and other similar bags, and I have no doubt it would last for years of use. However it probably won't get that, as I'll make another version after a while as I refine my designs and get it just right for my rack and bike. To start with, the top piece/flap is some generic 3 layer waterproof breathable fabric(about 4.7oz/sq yd), the wear areas that are against the rack will be heavy coated oxford (about 5oz per sq yd), and most of the rest will be 2.3oz silnylon. I will probably use some addition oxford on the front panels so that it is slightly stiffer and doesnt flap in the wind when the bag is empty. There won't be any rediculous leather edgeings on the bag (leather, really?), I'll use some nylon webbing to do an edge around the top flap. Much lighter, and I've never, ever, seen such edgings ever wear out. One would have to ride ze bike like 100,000km for 10 years at least to do START doing that.
Silnylon doesn't last forever...its coating wears off slowly (would be fine for 20 or 30,000km), but its very easy to apply new coating (I've done it plenty of times before). Also, all these fabrics are very easy to seam seal. It will be totally waterproof, without having to soak water into the fabric; it will mostly just bead off.
If I was going totally weight weenie, I would build it out of cuben fibre. Probably some .5oz stuff, with glued and taped seams, and some heavier stuff to reinforce against the rack, and maybe a piece of the 1.42oz material for the flap for breathability. The whole thing would probably weigh under 120grams. Materials more expensive though (but, cheaper than buying a Gilles Berthoud for sure), and it would be great for once of events like paris brest paris. It would probably only last 20,000km before the fabric starts to wear out...then you need to start patching it.