Only just read you post Gg, I'd use black plastic, my bin has a weed membrane but only as it was to hand when I made the bays.
I'm redesigning one of my beds and going to have a nursery for first sowings and seedlings. I acquired some pallet braces, I think that's what they're called, the plan is to have at least four small beds, close to the GH.
I'll take some pics and try to post them when finished. It may take a few weeks as the weather hasn't been ideal, it was snowing here this morning. Not sure what it's going to be like over the weekend. I've started to relocate the fruit bushes to make room.