"just" cover them?
Making an authentic-looking set of panels, with pockets, etc. would be a lot of work, and you would really need a special upholstery sewing machine. Cutting panels from scratch would be an easy part.
But if you want panels that are simple, just covered with fabric, it's probably worth getting them pre-cut.
BTW, I am in the middle of installing a nice mohair L-B interior, and I am finding that most of the panels are a bit oversize. On a lot of the panels I am having to peel away the fabric on one or two edges, and trim the cardboard down about 1/8" to 1/4". I cannot say if this is an issue with their panels in general , or just for my body style. Also, the thick mohair fabric (which seems quite high-quality, nicer than what Ford used on the panels) makes the problem worse. If I were going to install another S/W interior kit, I would consider having them send me the cut panels for me to test-fit, and I would trim them as necessary and send them back to be covered.
Doug