When taking measurements, measure the space that will be covered with your chosen sun protection product.
The main rule is: that you specify the size of the finished product to be manufactured.
Depending on which mounting type you choose, you may need to add a few cm to the width or height (for surface mounting) or reduce the dimensions by the same amount so that the finished product can fit into the intended space (for niche mounting).
You must make sure to make deductions to the width if necessary.