The cast on would get you the right width, and you could probably follow the pattern st after you determine how many sts in each repeat. There’s 6 on each end in garter… so you subtract 12 from the CO number. It looks like you need a multiple of 20 (that’s the k15, p5 that you repeat) plus another 15. I think 187 sts would work, it would make it about an inche wider, but you could do the same st pattern as in the original without adjusting.