98p large B&Q roller tray and a few bits of 3M self-adhesive velcro.
Warning! The roller tray plastic is very brittle and if you try to cut it without soaking in boiling water, it tends to shatter.
I soaked the tray with boiling water from a kettle, then cut out the only flat section.
While it's warm from the water, cut it into rough shape then apply more hot water or gentle heat from a heat-gun (a hair-dryer should do). Keep shaping it and it will remain pliable after enough heat.
Simply trim to fit and a few strips of velcro holds it in place.
I wondered if it would interfere with the sat signals but no problems so far.
For BMW Nav II owners, the rear clamp bracket actually holds the shade in place as well.