I have the factory GT3 (1st 996 generation) front bumper but haven't installed it. Call DEVEK Porsche and see what retention pieces are needed but Many people have it on their Boxsters as the full aerokit option.
I think for the money paid the fiberglass aftermarket bumpers are way way over priced. You're better of going with the Polyurethane factory which will survive a small impact. I have fiberglass side skirt but only because the GT3 (2nd 996 generation) wasn't available from Porsche for the boxster. I got them from Precisionporsche.com and they took about a month to make $430 deliverd. Fit is ok but not quiet factory there are tiny gaps where the rear mud guards used to go. But overall its the best visual upgrade after 18" Carerra wheels.
here's the GT3 (1st 996 generation) with GT3 RS inlets cut into them.
If you want the mesh from the factory to use as a stencil the part number is
996.505.325.91.
try
www.carnewal.com