Anthony Axford delivered a batch of quarter-boards of chipboard and some aluminium angle which I've used to board up some of my attic. Using the aluminium angle as guide rails I've been able to put down boards to create useful storage space while still allowing me to lift the boards up in case I need to do any electrical work in the future. The south side of the attic already contained quite a lot of wiring which I couldn't cover in case of overheating, but the north side was free of it so I've been able to create quite a lot of useful space there.
Happily, a quarter sheet of chipboard is exactly the width of the rafters in my attic. I didn't know that would be the case when I ordered them - it could be a coincidence but it's probably something deeply entrenched in ancient building codes that makes things a multiple of one foot sizes.