In Shakespeare's Henry V a line mentions the Welsh troops celebrating St. Tavy's day by wearing leeks in their monmouth caps. A later 19th century mention of the Welsh Fusiliers talks about them also wearing leeks in their busbies, as well as decking out their mascot goat with red and blue ribbon rosettes. Gingerbread "tavvies" were also sold in the 18th century, supposedly made up to look like a Welshman. But I'm pretty sure there's not much detail you can put into a gingerbread man.

I don't know of a specific Welsh costume for men or women in the 17th century. References to the women's clothing begin appearing mostly in the 18th century with them retaining some old elements of fashion into the future years. Not much is said about the men. I know of some surviving Welsh clothing from the 18th century. There's also this satire from 1750 of a Welshman. The only thing out of place, other than the goat, are the leg wraps, the blue rosette, and what may be an attempt at leeks on his hat. A similar image but of a Welsh woman and child have the leeks as well. Her petticoat hem being checkered is unusual, but the clear emphasis is the goat and the leeks. This broadside is also very close in style, again with the blatant leeks. With both the rosette and leeks mentioned in other texts about the Welshman's celebrating of that holiday, I'd probably conclude it was less about the clothing the effigy was wearing, and more about the details like that that make it Welsh.