Elle, that latest box is absolutely delightful, well done, you are making a great job of them. Corrugated is OK, but as you say, makes for a heavier box, but that shouldn't detract.
Brush, Matt board is a great card to use, will make a really sturdy box. And don't worry too much about getting the hex's in fabric exact, because all the edges get wrapped to the other side of the card and stuck down, or in the case of the outer lid, the edges get covered with the brim covering, so be sure to leave yourself a good allowance, (at least half an inch, but I tend to leave three quarters of an inch if I have enough fabric.)
Pal, I use an old blade in a rotary cutter to cut my card, it works a treat. I always mark the old ones with some nail varnish so I know which is which.