Why not start out with a square of fabric, either 40 or 44 inches? Do all your embroidery on the fabric and then cut it down to the size you want. Or do these as a reverse applique on the fabric where you actually cut the fabric out behind the embroidery. Your embroidery is very pretty.
A couple of videos on multi hooping.
https://www.youtube.com/watch?v=DSQF5ekK9ys
https://www.youtube.com/watch?v=iXtPlfODiKo
How about reverse applique?
https://www.youtube.com/watch?v=odtUb58CTM0