57 Fuel Door in Bed

[candyx]

57 3100 with wood bed previous owner install fuel cell in back of bed underside mount dead center of bed behinf rear end.It has a 3" pipe and screw on cap above the wood bed.I'v seen where guys have a flush mount billet fuel cap or door mounted to the wood bed which has a cap in the center.Does any one know what products to use or manufacture that sells that unit?