The angles aren't really that hard for the borders. Just cut the border strips a little long & mark the angles on the fabric and make a Y seam at each seam.
It's easily small enough to "birth" it to get it sandwiched.
I made a similar shaped runner but did the traditional sandwich & mitered corner binding on it. I had been practicing on potholders and coasters right before I did it. It's just a matter of lining up the angles--like billiards--LOL!