I bought my car with the cap missing, bought a genuine cap from a member on this forum but then found there's no way to affix it. I used to work for Subaru so I researched with my ex-corkers and found the clip to hold the cap does not come separately - its part of the main plumbing for the washers.
So I like your mention of drilling the holes and zip tying them on. How do you actually fit them? Do you need to remove the headlight?
Went through this a few months back. Looked at the exploded parts diagram and found where the pump was. No more streaky bonnet and no more lost covers.