mcrews — It's merely a Mercury skirt on a year-earlier Ford, with the stock chrome edging removed. Pretty standard in rockabilly circles.
Go around signage shops and see if you can get a partial (or used!) sheet of PolyMetal. The piece I got was 2x3ft or so for $20-25.
It's a stiff as 5/8" plywood. Follow the horizontal cut line for the bumper on top and box out the bottom like on that Miata. aerohead's example terminates in an angled skeg. That could in turn be an outer fence on a diffuser.