Re: new speakers don' fit
I recommend using plywood for baffles anyway. Plywood does not soak up water nearly as easily as MDF does, even if you paint it. Have you ever seen a speaker box that got wet? Lots of black mold and it swells + falls apart. Paint your plywood baffle after you have made all cuts, holes, etc.
Use automotive autobody sealant between the baffle and door. You buy it at automotive stores or even some well stocked grocery stores have it. Use sheet metal screws and screw the baffle to the door. Do your wiring and mount your speakers.....done! Takes about 2 hours and you will have a very solid foundation for your speakers.....which this alone will improve the SQ considerably over mounting into the existing holes.
'65 VW Bus
MA Hippo 122, (2) Aura Sound 8's, (2) SoundStream 6.5 coax, (2) Alpine Type-S 6x9's [rear fill], MBQ DSC1500.1D, RF 501X
'02 BMW 330xi (AWD coupe), every option
Harman/Kardon 12-speaker System Option + Dayton HO D4 10" and PPI Ion 650.1 in sealed box
And many more car audio goodies in storage