Variable width is unlikely to be a problem. At 3:5 the Scottish flag does not have an unusual aspect ratio. This is unlike the flag of Qatar, with a ratio of 10

Variable width is unlikely to be a problem. At 3:5 the Scottish flag does not have an unusual aspect ratio. This is unlike the flag of Qatar, with a ratio of 10:28, or Nepal with both a 3:4 (approximate, not exact) aspect ratio plus an irregular shape.