It may be that to do a wonky border for around a center straight block you need to have two border fabrics to cut and sew together. Sometimes it works to make the originial center block wonky also, but that doesn't sound like what you want to do here. When I make wonky (and I do pretty often) I sew the color strips I want together, then cut the border the way that looks best to me, making both sides "uneven", but the finished square exactly the same size as the others in the quilt. If you plan to use sashing you can cut it wonky also, but this is much harder than having blocks that end up the same size but with a wonky look.
I can't put a photo on but if you pm me with your email address I will send you a photo or two of what I'm saying.