Turned out I couldn't stand the big pile of unfinished bits and put it together completely without caring. First thing to go was the navy and white spot joining fabric - I did all the joins in scraps, so it wouldn't be so blindingly obvious that none of the joins actually matched up like they were supposed to. And then I just joined, and if there was too much, I cut it off. And I kept going, until I reached the edges which were all up and down like derelict battlements. And then I took my rotary cutter - didn't even both with a ruler - and sliced that sucker into something approaching a big, ugly square. You can see from the back how off it is - every one of those squares is supposed to be the same size and, well, a square.
And then I washed it and dried it and put it on a bed. I've made a lot of ugly quilts in my life. Even more that are badly made. But this one I loathed from the beginning. I gave it a very cranky label.