Well,
I went to Homebass, sorry Homebase, and saw the sheets of 12mm ply, much bigger than I needed and over £40, so I though "that's a lot to spend if it doesn't sound any good or i cock up the building bit", so I ummed and ahhed, and while I was doing this I saw some offcuts in mdf and chipboard, 90p and 60p respectively. That's more like it! Doesn't matter what kind of a balls-up I make of it for that money, then we'll see if if sounds any good, at least. The boy even cut them to width free! (did I mention I'm a tight git?)
I'll post some pictures soon showing the almost finished item, and the other money-saving "features".
I think it weighs 14lbs now.