Never be sorry when asking a question, how else can you get an answer!
I used the same material that they use on the back of cabinets, 1/8" press board. Comes in 4'x8' sheets. There was a barcode sticker on one of the sheets that indicated ABTCO, the bar code number is 6509609003 (if that would help).
I salvaged the original panel and traced the shape (including all clip holes and any other attachment characteristics that needed to be included) onto the 1/8" board and cut the board with a sabre saw (be sure to use a blade suitable for the material you buy) following the lines very closely (eliminates a lot of sanding).
I also salvaged the vinyl material off the old panels, by removing all the staples and carefully pealing it off. Then I took the material pattern to an upholstery shop to duplicate it (in my case I did the entire interior so some sewing was involved. If you are doing a flat panel you can cut a patern using the old piece as a patern and trial fit it to the new board. Note I used a piece of 1/8" foam padding on all panels (without it the material just doesn't fit or look right). You can use 3M General Trim Adhesive -clear product #08088 for attaching the foam to the board and then the material to the foam. I stapeled the material around the parimiter of the new board using a hand stapler and 1/16" staples. Flat panels are a piece of cake. Test fit everything before you glue.
I don't know what color your upholstery is, mine was red, and after looking for some time, I found a vinyl leather look alike made by Islander (item# 10950 at Bayer Frabics Louisville, KY 1-800-769-7779) which was listed as RUBY RED color #9160 that matched the original red very closely. If you are going to do only one panel, and you need to match the color, take a piece of the fabric that is not faded to a shop and start looking. Good luck on your project. E-mail me if I can be of more help to ya. I'm Havinfun