Now we're getting somewhere. I've added webbing and a shoulder strap. First I created a cradle of webbing to contain the dry bag. Tacking a length of webbing along the length of the grab-strap at the base of the dry bag . Then running a second length of webbing horizontally around the bag and tacking this to the first length of webbing such that the ends of the first length run vertically up each side. The bag sits loose in the cradle only being attached by the stitches through the grab strap. I sped things up by raiding and old camera bag for the removable shoulder strap. This strap was attached to the bag using side release webbing buckles. I stitched female halves of two buckles to the ends of the webbing running up the bag sides. The male halves attached to the borrowed strap clip in neatly converting the whole to a shoulder bag. Hopefully, if the text doesn't make sense, the two photos will help.
It's not finished. Not at all. I've used bit's and pieces of webbing and clips I had to hand together with the shoulder strap from another camera bag. All of the strap components are heavier than required and in time will almost certainly be replaced. The shoulder strap in particular is over engineered. The webbing is wider than needed and it is adjustable in the length. Straps on commercial products, sold to customers of many sizes and with different tastes, need to be adjustable. My strap does not. At least it won't be when I know how long the strap needs to be. Side realease buckles are also probably overkill. Certainly so if, in practice, it turns out that the strap stays permanantly on the bag.
Still, the bag's now finished enough to put into service. Using it will provide ammunition for improvements in the future. Only by living with it can I get a real feeling for what should stay and what must go. I've been confident in the construction from the off but was never sure about how it would work in practice. Ease of access to the camera through the roll top being the biggest concern. It had a first run out today, on a walk with the kids in the dunes. Hardly a multi-day backpacking trip, but enough to give me a good first impression. It's fine. I carried it strung diagonally over my shoulder with the bag against my left side and a 25l day pack on my back. It carries well and I can access the camera without a problem. In that respect its not as convenient as the Lowepro toploader on which the lid can be thrown open and left open. The roll top won't stay open of it's own accord. However, unlike the Lowepro, it provides waterproof protection for my camera. Furthermore, even with the heavy strap, it's just 160g compared to the 330g of my old Lowepro, topload zoom. The trade off in accessibility is worthwhile on both counts. This will be my camera bag for the upcoming trip to Rondane.