First finished quilt top (!) and ISO advice

Quilt Top

I finished my first quilt top a while ago but the quilt has been languishing in my closet because I hit a snag. I bought Twin size batting, but after laying things out realized that the quilt dimensions are called Twin but actually measure larger than the batting "Twin". (Lesson learned! Check the finished dimensions before buying batting!)

I was able to piece together a strip of batting along one edge to make it cover the quilt top. That worked fine. But now the edge is wavy. I'm not sure how to fix it, or if I should buy a larger amount of batting and try again. I was thinking I could try to smooth it out as I quilt it but I'm not sure if that is a terrible idea. Or cut the batting and remove the excess fold and re-sew?

Any suggestions?

Here's the join - that worked well!

Not the best photo, but this is the edge where the batting is wavy. :/

Pattern is the Standing Tall quilt pattern from Cedar Quilt Co.

Thank you!!