Suggested template, dimensions not critical. Simply bend the curved end back one way for one side, and the other way for the other. The tube hole is offset to the rear as the width of the strip is limited by what I can get out the 60mm width of UPVC square downpipe, but still keeping the clip towards the front of the seat-back.
Attached to the bottom of the headrest if it pulls right out (which the driver's does).
If it doesn't (like the passenger side) cut a small slot in the end so it can be pushed onto the tube, and you should still be able to reach the screw with the headrest pulled up.
Quite a neat installation, especially the later black version
Some time later something very similar appears in MGOC Accessories, for £27 ... now I wonder where they got the idea from ... and subsequently there are several reports of them breaking. There has also been an eBay pair made from sheet metal with a rubber grommet the head restraint stalk goes through, also expensive at £28.50. Currently unavailable, and I was concerned that with inertia reels at least the rough edge shown in one of the pictures would fray the belt with the continual to-ing and fro-ing.
November 2017: With Vee's new seat covers, she needs them as well.
Subsequently modified as with Vee's inertia reels much higher compared to Bee's - being on top of the rear arch instead of down by the chassis rail, the belt was lifting out of the guide in normal use. The first modification flattened the folded-over part fully down, but that was inhibiting belt retraction so I spaced it back to give a parallel gap. That put the tip under the head restraint which made it awkward to get the belt in and out to tip the seat forwards, so I cut the tip back a bit as well. But they were still coming out while being worn, and the driver's side broke because the sliding clip under the tongue was too high so the tongue was pulling the guide back when released. So I took the opportunity when making a replacement to make them higher with the protruding part nearly vertical. Early indications are that is successful, so I reshaped the Navigator's side as well: