I have a G35 Sedan 03 with Bose.
I was taking apart my door and trying to see how hard it would be to install new speakers took me about 10 mins to take it off really easy. Everywhere I searched on here says I have to make a bracket for aftermarket speakers. I took some pics below. Shouldnt I be able to put the aftermarket speakers in the white plastic bracket and just cut out the back.

 

You can modify the existing plastic brackets but the time it takes to modify it, you might as well cut new MDF brackets (spacers). New MDF spacers are less likely to rattle with the added bass from aftermarket components.

 

i have infiniti kappas and i didn't need any brackets

 

LIke posted above, it is easier to just cut rings from MDF. Plastic also resonates, which is a sound that I can personally live without.