Well I ordered a Zeatek one - if i had a template in the shape of an old plastic tray, I'd have probably made one myself, but sometimes you just need to pay the money. Anyway, it arrived from Denmark in 2 days. It went on easily, and I also used M5 Hex sets with penny washers and plenty of grease on the threads.
There is no obvious difference when driving, I didn't get in the car and think "wow, this is so much better". But, I have noticed two things:
1) That my door seals, which previously never bothered me, are now annoying me because of the wind noise. So presumably at speed, it is making a difference to the noise, and allowing me to hear the wind noise from the (failing) door seals. I would imagine that it would make a huge difference to the wind buffeting on the bulkhead, so this does make some sense.
2) The MPG seems to have improved. Normally if I drive it hard, I get 300 miles on a tank. If I drive like a nun, I get 360. The last tank I drove hard - 80 - 90 on the motorway, not really worrying about consumption, and I got 330 without the yellow light coming in. Same commute as normal. So only one data point, but seems better.