So I took the slip stitching out and then crocheted the tried and true joining and border that I love.
I'm still not sure if I'm done or not - I'm thinking I might add one more row. Right now it measures 27" x 33 1/2". For the squircles I used various brands of worsted weight yarns - and for the joining I used Caron One Pounder. :)
Here's the link to the border I used: