So the plan is very simple. Each stack to consist of:
1) Two side panels of 4 inch plank
2) Three cross-bars of 4×2, the bottom one with the lower speaker resting on it, the middle one with the upper speaker resting on it, the top one to complete the box. Actually the middle one can be less wide to reduce weight – if it’s 4″ it’ll stick out at the back. As discussed, I’m using wood reclaimed from skips, as the builders around here seem to chuck out nice clean planed timber. It’s good to have the top, bottom and sides all 4″ wide though, makes a neat and workmanlike box round the speakers. Of course you could go wider than 4″ (at some cost) and achieve greater mass and stiffness.
3) Two 2′ lengths of wood as feet, running fore and aft for stability.
4) Two castors per foot for moving the damn things about.
If the cross-bars are a couple of millimetres smaller than the width of the speakers, the screws running into them can be used to pull the side panels in tight against the Quads, clamping them firmly into place. A length of 835mm is about right for this.