To be honest I hit a bit of a wall on whether this was the right way to do what I was doing. I need to think about it, but I will come in next weekend to sort out one way or another (it may be that these bits end up as - hopefully useful - scrap).
I also have some long pieces of stock of the same box section that I have two things I want to do with them - need to think about it a bit more but will make some progress soon.