We generally think of trews as high-waisted, close-fitting pants, probably because we generally see them in period pieces or in older artwork. At the time that trews were more generally worn, I think all trousers were of basically the same high-waisted, close-fitting pattern. 'Trews' is just that weird Scots way of spelling words the way they pronounce them and pretending that it’s a different word. Trews are trousers. So if you want trews of a more modern design, then you want tartan dress slacks. Nothing special there other than the material. Quick searches in Google and Yahoo didn’t show any photos of trews with the material cut on the bias.