This isn't a part that is readily available from places that sell Ampeg replacement parts. Why not go to a music store and compare what you need against one on a current SVT-CL. Then contact Loud and see if they will sell you one. Otherwise, people have made their own for original SVT's. There are a couple of threads here discussing it. It can get expensive depending on how it is done.
Official Ampeg Portaflex Club #89
If it's like mine, it's just a rough-cut section of expanded metal with a slight bent edge at the bottom. If LOUD can't help you, then you may be able to easily make one for only a few bucks at a hardware store (?).