Ah, clever idea with the chipboard.
This person on eBay is selling all sorts of sizes, e.g. 210 x 297, 250 x 250 (mm here, just remember that 305mm = 12"). That's in the 3mm width. There's 297 x 210 (apparently that is A4) in 6mm, I wonder if I'd be better off with that to avoid distortion? Anyway, this is why I was wondering whether you actually needed the full height in order to see what was going on.