How can you use a old panel if you do not have a matching fabric?
If you can not match the fabric colour on the borders of the panel, cut the outer border off,
add a small strip to each side of the panel, (black works really good to frame the panel) then
pick a multi colour stripe or abstract fabric to enlarge the panel to the size you need.
The black strip works as a place where the eye will rest before going to the other fabric.