Since this afghan is already under way, there's nothing much you can do to make the start and finish look the same, other than crochet a line of single crochet along the starting edge.
However, for the next time, may I suggest you search for videos or written tutorials on Crochet Hook Cast On? It does create a cast-on edge that is identical to the usual cast-of, and it's nice and stretchy. Can be made moreso by using a crochet hook larger than the needle size.
The knowledge you have and are willing to share is phenomenal. The afghan I'm doing is in strips so it would have been an easy task to do the crochet hook cast on. As you say for next time. Thank you so much! Please hold my feet to the fire until I get this completed. :)