The other thought I had apart from the sailcloth side bags, was to get 4 kit bag/sausage bags from my local motorcycle shop-
They cost about £8 each and are waterproof. I would then cut off the sealed end off one of the bags and sew it onto another, making a sealed sausage bag at both ends. I could then add a zip down the middle and using the spare cloth from the cut bag, I could add a covering flap over the zip for extra water protection and pockets where I wanted them.
All I need to do then is add securing straps where I want them. I think it would work and be more waterproof than the radicals, at a fraction of the price.
If I give it a go over the winter I will take some photos as I do it and post them on here.
Steve