Having been under the car recently, I can now see where you're coming from with these fittings, Arran just threw me when he mentioned caged nuts .
However the stainless clips do the job just fine on the (very) top mountings which are unsuitable for caged nuts.
Out of interest, what will you use to bond those fasteners?
(hope it's as good as the stuff used by a previous owner on my front "bumper" - quite clearly it had suffered from a small split and been repaired - hasn't budged).
Anyways, wot with being in the process of stripping down and refurbing my D, I'm always on the lookout for good quality fasteners and "best practices".