It's time to share the result from my square pumpkin experiment. As you can see I definitely managed to get some flat spots on the pumpkin but I failed to achieve overall squareness (The first time in my life I haven't been square). I think I got to the pumpkin too late and was too optimistic about how large it would grow. I built a box just big enough to hold it when the pumpkin was pretty much at full capacity. I also let the vine grow a second pumpkin because I only have 3 pumpkins appear in total (very small patch).
So the plus points, the box worked really well. 5mm perspex was the right material and anything thinner might have cracked. The slot and peg design held together well and actually needed a bit of a whack with a hammer to get them out after 2 months. And the best point is that I still have an interesting shaped pumpkin to carve. I'll be doing it again next year but I think I'll make some inserts to go inside the box and just reduce the size slightly. (I'm also pretty sure that a whole year is not the longest time I've made you guys wait for a project, Donkey Kong revamp anyone?)