Measure the height and width of the panel top and bottom and at each end as well as the middle.Tape a piece of paper to the entire panel. Entire meaning the full width. Trace the panel on the paper with a sharpie. Remove the paper and lay it flat on a table. Now you have the outline of a 3 dimensional object flattened in 2 dimensions.
Either take a picture of the outline, being careful to keep the camera centered and parallel to the tracing, and import it into whatever software you're using and make it full scale.
Or draw horizontal lines from the top left corner to the top right corner, from the bottom left corner to the bottom right corner, one at the top that just touches the highest/lowest point on the top of the tracing, and one at the bottom that just touches the highest/lowest point on the bottom. The width and the distance between the two top lines and the distance between the two bottom lines will enable you to reconstruct the approximate curve of the panel.
Set your text into either the photo or the recreation and bend it to fit. Cut and apply. You will intuitively think it's not going to fit properly. Trust geometry, it will fit. Perfectly.