I'm making a secondary glazing panel myself (not with magnetic strips) out of plastic (acrylic or the likes of) and with a wooden frame around the edge to secure to the window. Some of the prices are quite reasonable (~£60 for a 1300 x 1800 panel), so I prefer the idea of this rather than the very expensive aluminium kits that you can get. Whilst doing this, I suppose I could create a double layered glazing kit e.g. two pieces of acrylic with a ~10mm gap between. Would this be even more efficient, or a waste of time do you think?