Also, "Congratulations for winning" sarcasm applies to ALL fantasy role play. If you are going to go there, you dont belong in this thread, IMO.
I'd actually argue that its inapplicability is foundational to roleplaying games in general. They're multiplayer for a reason.
If you want some fancy flying carriage made of immovable rods as cool background flavor, great -- but as a DM, you can just
do that; you can just put it in the game and let the players ooh and aah at it and there you go. If you want to spend a lot of time detailing to yourself how it works, that's fine too, but the thing is that unless they open the hood, that information won't enter the consensus understanding of the game, so in a functional sense it was never real, however true it is. It's the same way that you can spend ages detailing what's in the castle three kingdoms over the players are never going to visit and justifying it down to the last gold piece. If they don't ask, it's not in the shared construct of the game. It's just world-building solitaire.
Similarly, if as a player you want to build the thing, there's a question of whether the rules minutae matter. If you came to my table wanting to build it, I'd stat it as a modified carpet of flying and compute the price from that, and the details are up to whatever you think is cool. Immovable rods, enchanted swans, a boat that floats in a tiny flying ocean, whatever; it works how you want. Go nuts. If you want to be able to pull the rods out of it for other purposes, then yeah, it's going to cost as much as that many immovable rods, but ultimately whether or not the rules support the literal nuts and bolts is immaterial. Again, nobody's stopping you proving to yourself how it works, but ultimately they're not going to dispute it, either. It's a prop in the story everybody's creating together.
I'm not saying it's necessarily a waste of time to establish in detail how it works by fiat; world-building is important, after all. I'm wondering, earnestly, if there's ever a context in which it needs defending on rules grounds instead of "I made it and we're all okay with it", whether from a player or the GM.