A quick look did not come up with pictures of the old system but it was a bit like a large plastic mudguard but with sides with attatchment points for stretchy netting that held a dry bag, think it snagged and broke a lot
I'm working on some horse saddle bag/panniers, not cycle ones obviously... but... I'd imagine a good approach would be to use a mesh front sewn to a back made of light poly oxford cloth/rip stop, maybe even sew in a removable pocket so you can slip a piece of foam in for rigidity against the pannier rack. All you'd really need on top of that would be webbing loops/fittings to attach it to the rack. I have a few ideas for fittings/attachment, but a good route could be using sail fittings. Drop me a line if you want to swing a few ideas around. It's all fun to me!