Update:
I gave it a go. I f**ked it up in several different ways, but in the end I think it turned out ok (given my very low expectations 🙂)
The biggest thing I got wrong was not realising that even though the sides look like equilateral triangles when you look at them from the ground, because of the 30 degree slope, they're actually stretched out taller than that when you look at them flat. I used equilateral triangles, so I ended up with a squished pyramid...
Some pics:
And I put some lights in there to snazz it up a bit too:
In case anyone wants it, here's the powerpoint I used to get the (fundamentally inaccurate) dimensions of all the bits of cardboard: https://www.dropbox.com/scl/fi/455f13wnnbnlw0i9tsv3u/pyramid.pptx?rlkey=cm8mjcb5eq7g3oi890tm0spp4&dl=0
Thanks to everyone who helped!