The 5e version of the plane shift spell is continuing a long tradition of using these forked metal rods as part of the spellcasting. A spell named plane shift, used to teleport between different planes, and requiring forked metal rods, has been present in every single edition of Dungeons & Dragons, with the sole exception of the very original, and 4e, which was very different in many regards.

Furthermore, the exact planes that exist and how to get to them is a setting detail: they are determined more by the setting than by the general rules. So far, all of the settings published for 5e have been settings that existed in prior editions, however, so in this regard 5e can be assumed to be maintaining the same setting details as in prior editions, excepting when 5e specifically calls out changes to them.

Thus, in short, my answer to your question is that the plane shift material components work the same in 5e as they have always worked in prior editions of D&D, that is, the forks are “attuned” through careful selection of materials and precise construction (originally, the forked rods were explicitly tuning forks, so they were very literally “tuned” to a frequency particular to that plane). They have never been magical (and indeed, in 5e, spell material components are not generally magic items), and they have not previously required that they be made of material of the destination plane. Therefore, yes, based on the history of the spell and the default settings used in 5e, you can create a focus for plane shift with the 50 days of downtime.

The real significance of the attunement is that a given rod can only be for one specific plane, chosen during its creation; to go to a different plane (say, where you came from), you’ll need another rod specifically crafted for that plane. On this point, the 5e Player’s Handbook implies the same functionality as previous editions, but it isn’t as clear as it could be: when it says to “specify a target destination” or “the plane of existence you specify,” you specify the plane by using the corresponding rod. When transporting yourself and others, you can indicate a more specific location on that particular plane (as described in the spell), but the rod still has to match the plane you want to go to.

But this is a setting detail, and the DM is in charge of the setting. 5e is no different from other editions in this: the plane shift spell itself has never described the construction of the rods in detail. When playing in the “canonical” versions of “canonical” campaign settings, you can create the rods without traveling to the destination plane first, but the DM can easily change these details.

As an aside, the fork being attuned with a plane has absolutely nothing to do with characters being “attuned” to magic items. A focus is not, itself, magical, it’s just used for doing magic, and in any event, the description of plane shift never says that any character has to attune to the fork; it says that the fork has to be attuned to the plane it’s to be used for.