the R33 that i put the Alpines into had the factory pods, we made a 6mm speaker ring to sit inside that. took 10min max to make a cardboard template, trace that onto the MDF, cut the MDF, sand the edges to get rid of the flash, and screw straight in with the speaker - into the factory holes.
only reason we took the factory pod off was to put sound deadening on the door and inside the door.