front 16v bumper smoothing...

Hi everyone and to the point lol...i have got a 16v bumper side vents grills all smoothed of already, and not attempting the number plate recess. i understand that it has to be plastic welded cuz its the best way. I was confused on if to use an old front bumper or a rear bumper to cut out a section which has to be welded??

which one would be best to use?.. I've cut out the recess off the 16v bumper alredy hope thats the right thing to do:P

(if i cut the section from a old front bumper it seems that it will not cover the number plate recess of the 16v bumper...so 2 sections might have to be put togther)

any insite on this? and can anyone provide pics of the process?, it might help lol

thank you v.much :-D