nice custom kit 
if it were me, i'd leave the metal piping in there (mine has a mashoosive metal cold air feed https://www.fiatforum.com/gallery/data/500/medium/2107cold_air.jpg but if you did want to swap the metal for plastic stylé tubing, then these are the part numbers of what you'd need;
2 inch silicone joiner from SFS performance (SCH51) £3.92
2 to 3 inch reducing elbow from SFS (RE90-76-51) £22
http://www.sfsperformance.co.uk/
if it were me, i'd leave the metal piping in there (mine has a mashoosive metal cold air feed https://www.fiatforum.com/gallery/data/500/medium/2107cold_air.jpg but if you did want to swap the metal for plastic stylé tubing, then these are the part numbers of what you'd need;
2 inch silicone joiner from SFS performance (SCH51) £3.92
2 to 3 inch reducing elbow from SFS (RE90-76-51) £22
http://www.sfsperformance.co.uk/