I bought my present bike with panniers already lined with dark "carpet" used in car interiors. Relativly cheap and hardwearing, and not too difficult to cut and shape, glue with contact glue, according to Ulf who sold the Geezer. Now I have new panniers from the insurance after a knock down in oktober last year, got the bike last week, after six months of stinginess and procrastination. Now I will have to do the job.
One modification, take a thin plywood plate a few mm smaller than the base of your pannier, drill a fingersize hole, cover it with your carpet with a few mm overlap, make an X over the hole, and you have a false bottom. I am sure you can think of something to hide under there.
Peter, in Oslo