Okay so, after a few hours questioning my knowledge of basic geometry, I've made a pattern for Lalli's hood/hat (or at least the base).
it was actually deceptively easy... Unfortunately I can't post it because I made it out of an old newspaper so here's some instructions I guess:
First, find a jumper with a hood that's around the size you want. Fold the hood flat along the seam and draw around it.
Mark off a third of the way down the "open" edge of your traced shape. This is where the triangle bit will come to.
Draw two diagonal lines from the bit you just marked and the top of the open edge to an equidistant point roughly 10cm away.
And if I've explained this clearly enough, you should now have a shape that will be half of the white base of Lalli's hat/hood. I can't put the picture of mine up at the moment but it's on my tumblr
here.
And now it's time to stress about earflaps.