If its any help, when I fitted mine I just trimmed the existing lower cowl using a dremel, with the 'mini milling' cutter type bit fitted (not sure exactly what they properly called !) .
On the inside of the cowl there is a moulding line, I just trimmed it to that and it fits fine, probably better than I expected to be fair ! I thought its worth a go and if I f**k it up, then I'll just have to buy the preshaped one to change at a later date.