May take some playing with but with straight needles, CO number of sts it takes to get your desired bottom or top width on the oval. Then increase for example:
Row 1: K3, M3, k6, M3, k3
Row 2: k1, p16, k1 (the k1 will help keep it from curling)
Row 3: K4, M3, k8, M3, k4
Row 4: k1, p20, k1
And so on. Do this until desired width is achieved, then begin reduction in the same manner only opposite.
Row 13: k4, sk2p, k8, sk2p, k4
Keep in mind that I'm just showing sts as example.
Hope this helps! Mary