Think "Big Picture" Ditch! Do it right the first time and you won't need to go back to it!
Concrete all the way. You can mark out, add ballast and shutter your bay at your leisure you said so no imediate time constraints.
Bit of weed killer and some visquene sheeting or similar first. If the base is, say, 3m by 3m and about 100mm thick you will need about 1 cubic metre of concrete at about £100 (English). When you order, they'll ask you what it's for, ie driveway, foundations etc. Get a mate to be there when it arrives, you'll need a buiders shovel or two, a piece of (at least) 3" x 2"