I got tired of my parcel shelf falling down so about 8 months ago I purchased a redesigned set of fittings from eBay, but even those failed as the plastic is not strong. So here is my alternative solution costing less than $12 in hardware.

Parts
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Instructions

• Remove the parcel shelf and all its original fittings.
• Remove the liftgate upper trim panel using a trim tool to release its 12 clips (see 76.34.11, page 3180 of the Jaguar X150 Service Manual).
• If desired, paint one side of 4 fender washers black.
• Bolt the parcel shelf to the liftgate trim panel using four #6-32 x 3/4-inch machine screws, four #6 pan screw covers, eight 1/8 x 1-inch fender washers, and four nuts. Sandwich the parcel shelf and liftgate trim panel between two washers as shown in the picture below. Tighten the nuts being careful to not overly compress the parcel shelf’s fiberboard. Note that if you have not already drilled holes in the parcel shelf’s fiberboard for the forward two fittings as required for the redesigned fittings, you will need to do this. I suggest bolting the parcel shelf to the rearward two liftgate trim-panel holes first, then mark and drill for the forward two fittings through the center of their liftgate trim-panel holes.
• Flip the pan screw covers closed to hide the screw heads. If you center each of the machine screws in its liftgate trim-panel hole, the nut and excess screw length will fit inside the liftgate’s hole that the original parcel clips went through.
• Reinstall the liftgate upper trim panel into its 12 clips.