If you are looking at the "build-your-own" on Freeserve, one of the key bits in the plan are the PVC cross pieces that help form the spine. Although the writer says they are out of production, something that looks similar is listed on lbsgardenwarehouse.co.uk (four-way tee listed in three sizes). I have spent ages looking for a cross piece and its the only one I have found so far. I don't know if it will work, but it looks a possibility.