dont ever think about cutting the metal or plastic
u will ruin ur car..
u can add 4 6.5 inch speakers in doors... get good ones.. like pioneer d series , alpine type s or r and amp them u will be good to go..
or the best option is to get components for front and a sub... no need for back speakers..
or u can simply get components for front and normal 6.5 inch for back doors...
u will get amazing sound , it does not have to be a 718
and u will not damamge ur car..