Quote from Kueatsu >>

What would be a survivable plane?

 The Ethereal would work well, if that fails then use the Astral or an upper plane, if you have a big ship then Water might work.

There much precedent for how a forked metal rod gets attuned to a plane, or no? If not, anyone have any suggestions, say, in case a player asks about it, maybe even aims to make one personally?

Quote from Dartagnan_Galochio >>

There much precedent for how a forked metal rod gets attuned to a plane, or no? If not, anyone have any suggestions, say, in case a player asks about it, maybe even aims to make one personally?

 There’s no official rules for getting the component, as far as I know. If you want your players to work for it, I’d suggest using rules similar the Magic Item Crafting rules from Xanathar’s Guide to Everything, treating the Tuning Fork as a common or uncommon item. For the special ingredient of the crafting, you could have the players face a monster related to the plane they want the Fork attuned to; for example, a Phase Spider for the Ethereal Plane, a Slaad for Limbo, or a Githyanki for the Astral Plane.

A thought: If you cast Demiplane, and took the time to craft a tuning fork attuned to your demiplane (assuming for the moment that you could) is there any reason why you couldn’t use Plane Shift to instantly go to your Demiplane without casting it and opening the door?

In order to cast this spell, does only the caster have to be level 7, or the whole party? 

Quote from Leo_Ultra >>

A thought: If you cast Demiplane, and took the time to craft a tuning fork attuned to your demiplane (assuming for the moment that you could) is there any reason why you couldn’t use Plane Shift to instantly go to your Demiplane without casting it and opening the door?

 I'm a bit late to answering this, but I don't see why you wouldn't be able to use Plane Shift (or Gate if that becomes an option) as a substitute for accessing the demiplane.

Quote from GrahamBladestorm >>

In order to cast this spell, does only the caster have to be level 7, or the whole party? 

 The caster has to be able to cast this spell. The rest of the affected creatures do not. This is a 7th level spell, however. A full caster does not have access to 7th level spell slots at 7th level. They first gain them at 13th level.

tarrasque killer

If this is granted as an epic boon due to an Aberrant Dragon Mark, can it appear on a character’s sheet?

Is the Rod consumed by the Spell?

Nope! If a spell's material components are consumed, it'll say so. The components for the revivify spell, for example, are "diamonds worth 300 gp, which the spell consumes". Otherwise, the component sticks around and can be used for subsequent castings.

thanks

"tarrasque killer"

good luck landing that melee spell attack, even if you have the rolls as a divination wizard, getting in and getting a turn is pretty tough with 120ft blindsense

So what would a tuning fork for the prime material plane be? Is it just a metal rod that is given any enchantment that would make it worth 250gp? If a player was wielding a +1 trident would that work as a focus or is there something special to make it actually attuned to the PMP?

Unlikely. The plane of water has no surface. Only endless water. You would need a submarine.

Beholders are from the Far Realm, not the Abyss.

Per the DMG on the Plane of Water: Any land that rises above the surface of the sea is hotly contested by the few air-breathers that live on the plane. Fleets of rafts and ships lashed together serve as solid ground where nothing else is available. Most natives of the plane never break the surface of the sea and thus ignore these habitations.

The plane of water having a surface is a new description for 5e. In previous editions it had no surface, only infinite depths. But ultimately what the water plane is actually like is up to the DM and the world they're running.

Is there any 5th edition clarification that you don‘t need just one fork attuned to your home material plane, just like a personal compass showing north for your personal extraplanar orientation?

Please no argument like „It worked like that in previous editions“, RAW here in 5th I see absolutely nothing that says otherwise, especially considering the Banishment part of the spell where I can „specifiy“ a plane of existence.

I did a bit of looking here on D&D Beyond and I didn't find anything that specified one way or the other. I'll look a bit more latter and update my post if I find anything. (If I missed something I would love to know what it says, people reading these this comment)

I do want to point out that it does say "...attuned to a particular plane of existence". What "particular", means would open the interpretation of your group's DM. Personally I would say that you would only need one attuned to your home plane to act like compass, like you asked about, but it would also require some knowledge of how the cosmos is laid out in your world, and maybe of the plane you want to travel/banish to as well. If your curious about how it works in your game, or want to take the spell but are wondering what kind of stuff would be involved in making it work, just ask your DM.