Altering Sub box to fit new sub
 

I got a new car with a custom box fitted for the car, the previous owner had a 13" sub in it before. He left the box but took out the 13" sub (part of the deal when I bought the car). I have a 12 inch sub that I was planning on using but the cut out is just a cm too big to mount the screws for the 12". I'm trying to figure out how to add that missing MDF. Any ideas? and no I dont want to buy a new 13" sub.

I was thinking of cutting a 1 cm thick circle, mount that around the old cut out with construction adhesive and screw some screws sideways into the existing box.

Let me know what you think.

Something larger and mounted over top or underneath would be the best way. Screwing into the ring sideways will cause the mdf to split. Also the weight of the subwoofer will be too much and cause it all to fall apart.

I was thinking doing what you suggested underneath.

Yeah, an entirely new baffle (front panel) would be the easiest and best way to do it. I can do it for you at the shop.
Another thing to consider is the size of the box (volume) and how it will work with the new sub. For example, if the old sub was a JL 13W7 and the new sub is a Hertz 12", there will be a considerable difference between the two specifications for optimal volume. The W7 will take a 1.95 cubic foot sealed and the Hertz will take closer to a 1 cubic foot or less sealed. Having a box that is too big will affect the performance and longevity of the sub.

I checked the Volume and the box is 1.578 cubic feet and the sub recommendation was 1.25 would that make a big difference in sound quality?

Thanks for the offer but I got some power tools I've been waiting to mess around with so I'll give it a try, but if I mess it up all call you to make me a new box lol.

The sub may 'bottom out' at that size. It should play lower with the larger box but it could be a little risky at higher power/volumes.

I dont really plan to pound it. I'm out of that stage where people down the block can hear me. I wanted to add the sub cause I do enjoy some bass plus the box is already there. So at mid levels you think it would be ok, sound quality wise?

no worries with the box too large you can always add some material to the inside to reduce the volume accordingly. Or...add an external aeroport since the enclosure is already a little large for a sealed enclosure it should be about right for ported. Just have the proper diameter and length port for your desired tuning and I bet you'll enjoy the music even more than sealed.