Design Space only allows a 11.49" image so we need to "trick" Design Space to make a full 12" border. This Simple SVG features a full border but it is not a 12 inch border.
But notice in the photo, the border is much different! Be sure to watch the YouTube video to see all the details but here are the main steps:
Ungroup your Simple SVG and select the 2 border layers.
Group and change the width only to 6" leaving the height to adjust accordingly.
Duplicate this image and cut two.
Together, they will give you a complete 12 inch border.