Sharing some stuff here since I received quite a few queries pertaining to pickup covers / cover installation. Do note that the pickups in question here are passive Seymour Duncan Nazgul & Sentient humbuckers.
Do the covers (regardless of the materials) impede performance? Thusfar, there have been no records / documentations to show a significant difference in performance between covered & non-covered pickups. The pickup's magnetic field are strong enough to penetrate the cover materials. If there was ever an anomaly that significantly affected output / performance, companies like Gibson for instance, would have given up covering their pickups a long time ago.
What are the supplementary steps to be taken after covering pickups? If you are covering your pickups like how the manufacturers are doing it, wax potting would help prevent microphonic occurrences. If you are doing it for novelty reasons (like yours truly), it's a mere covering of the bobbins & pole pieces; drop in & ready to go.
Will a typical pickup cover fit into any humbucker? From my personal experience, this is the case with the flapless version (refer to the top-most pic of this post). As for the flap version (depicted above), the length is not universal. Wider pickups, eg. Seymour Duncan Trembuckers, would manifest minor fitting issues. FYI - SD Tembuckers feature a longer bobbin vs a DiMarzio F-Spaced pickup. DiMarzio keeps the bobbin length the same for both standard & F-spaced versions, only the pole pieces spacing are different.
What are the advantages of covering pickups if this doesn't affect performance? 1) Covered pickups are protected from the player's sweat or other external moisture interference. This way, the pole pieces won't rust / oxidize easily. 2) Covers also protect the pickups from pick scratches. Some of us tend to scratch the coil tape as well (the brute players especially) so the covers protect the tape & coil wires. If you intend to sell off the pickups in time to come, covered pickups would look more presentable once the covers are removed (obviously, duh...). 3) If you are bored or experience a surge in artistic assimilation, you can doodle on the covers with paint markers or simply paste stickers on them.
How much? Plastic covers are $3 each (average price), metal versions typically lists for $5 each or more.
Bottom line - covered pickups are neither inferior nor superior to their uncovered counterparts. It's a cheap, reversible, fun mod.