The kilt first appeared as the great kilt, the breacan or belted plaid, during the 16th century. The filleadh mòr or great kilt was a full-length garment whose upper half could be worn as a cloak draped over the shoulder, or brought up over the head. A version of the filleadh beag (philibeg), or small kilt (also known as the walking kilt), similar to the modern kilt was invented by an English Quaker from Lancashire named Thomas Rawlinson some time in the 1720s. He felt that the belted plaid was "cumbrous and unwieldy", and his solution was to separate the skirt and convert it into a distinct garment with pleats already sewn, which he himself began making.
https://en.wikipedia.org/wiki/Kilt

The belted plaid was a standard item of men's Highland dress from the late 16th century until the middle of the 18th century. It was also the precursor of the modern tailored kilt.
https://en.wikipedia.org/wiki/Belted_plaid

descrizione: spalle (schoulders), plaid and shirt (sciarpa di tartan e camicia)

plaid = tartan, tessuto con motivo scozzese a quadri, tessuto costituito da strisce orizzontali e verticali in diversi colori, su uno sfondo colorato

Contesto
The highlander had been xxx too, she thought vaguely. Though entirely xxx and he had no soft, inviting twinkle to his eye.
His shoulders had stretched xxx wide, testing the width of his plaid and shirt.

Foto: ´Sciarpe di tartan ...
Che cos'è il tartan?
È un tessuto costituito da strisce orizzontali e verticali in diversi colori, su uno sfondo colorato
Le strisce intrecciate si chiamano "sett"
Originariamente i vari clan usavano piante, muschi e bacche locali per tingere la lana
Esistono oltre 7.000 tartan unici: persino gli Obama e Hello Kitty hanno il loro tartan! ...
Il tartan oggi
Ma il tartan non è una reliquia storica: ancora oggi svolge un ruolo nella moda in Scozia e nel mondo. ....
È impossibile immaginare le sfilate di moda e le uniformi scolastiche senza il tartan. In tempi recenti, alcuni dei più famosi stilisti di moda hanno utilizzato questo tessuto, con i suoi colori sorprendenti e il suo legame storico con la ribellione....
https://www.visitscotland.com/it-it/things-to-do/research-yo...

Tricky, but I think you would greatly confuse Italian readers. The setting of the scene makes me think this is Romance. "Abito scozzese" might kill the vibe, so to speak.
Whilst "plaid" differs from "kilt", it is the original form of kilt, and I would be tempted to translate it with "kilt" given the context (even if kilt is only the modern version of the plaid and differs from it). Plaid is "il grande kilt" i.e. the original, large, 1-piece garment. Hope this is clear

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Note added at 27 mins (2023-06-30 08:16:02 GMT)
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Got, it! Then, I would go with "mantello" or "tartan" even if "tartan is not 100% equivalent to plaid (it's a type of plaid), given that the author refers to the plaid stretching over his shoulders.

In un dizionario risulta che per plaid si intende tessuto o stoffa in stile scozzese, quindi a quadri.