We have used decking on the Ruschbahn and on my own line I have used Wooden Gravel Boards (from Wicks). Both work fine if you make them up to to a U (well not actually a U) by using 2 boards for each length of baseboard. RIP the second one and use the 1/2's to make up a Half Box (U) section with Skrews. Cover with Roofing felt and you have something that should last for years and years.
Another option is to get hold of Plastic Decking, this stuff is everlasting. Special requirements for cutting tools. But slow work with a hand saw will be fine. You can also use this in DIRECT CONTACT WITH THE GROUND. So it is a good, but not as cheep option. But when you look at the amount you are going to use it does not work out that bad. Certainly cheaper than concrete ground level foundations (and easier on the back). You would still need to think about additional support for an above ground installation.
Search on Google for Plastic Decking. The link below is one of many that I found just now:-
http://www.timbertechuk.co.uk/composite-decking?gclid=CKrr2sjb_p4CFU0B4wodtGTfIQ < Link To http://www.timbertechuk.c...2sjb_p4CFU0B4wodtGTfIQ
Good Luck and let us know how you got on.
JonD