Just on the difference between drilling a cap and poking a piece of aluminium foil, the generally-accepted theory is that foil is best, because thickness matters a lot. Including the sanding, that's designed to make it as thin as possible. Here is a bit of an explanation of how it affects angle-of-coverage. I thought there was another effect on other things but my google is failing me, or maybe that's the only problem.
I'm thinking of my lens caps, they're generally 1-2mm thick, way too much for a real hole. If you weren't already going to, I'd drill a 10mm or so hole in the lenscap and attach a piece of foil or otherwise to that...
(Or take the lazy way out and buy a Skink Pinhole kit like I did)