I like the braid border idea. You can certainly use 2.5" strips. I would experiment with scrap fabrics to figure out how long to cut the pieces. You will need to add 10" borders on each side. Given the design of the top you have already made, I think it would be best to enclose the braid on each side with a plain fabric border, so you might want to make the braid itself 6" to 8" wide. Here is a link to a Crafty tutorial on how to make a braid:
https://www.craftsy.com/blog/2014/12...h-braid-quilt/
If you are short on fabric, you might want to cut the pieces to exact size before sewing so you don't have to trim.