Usually you just cap the originals with 12 ga mild steel.Jack it up and have at it with a tape measure.Sketch up what you think you need,all dimensions should be shown as inside dimenssions because whatever you have made will need to fit over the old rails.This is the easy way and will last for a while.The better,harder more expensive way is to cut the old rails out and have them duplicated.I make rail caps for 1st gen Z's and sell them thru "Bad Dog Parts" but I dont have patterns for a ZX.