MrSparkle – give it a go.
I bought 2.5m of ripstop PU Nylon (from Point North) and made my own. Simply cut it to size, hem the edges and sew on loops of webbing for fixing points. I also reinforced the fixing points to make sure they didn't tear.
It cost about £10, is 1.4 x 2.4m and weighs 220grms.
Next project is to try and convert a dry bag into on of those Revelate seat post bags. I've used a dry bag bungeed beneath the saddle but would prefer something a tailored so it doesn't hit my legs when I pedal.
Anyone tried to make something similar?