walling off an escort zx2

ok so i have an escort and right now i have 2 12s in the trunk and does nothing near what i want it to. plus i always wanted to try this project so no better time then now. my question is how to make the wall? can i just cut a peice of 3/4 mdf and "snug fit" it into the backseat area? also how would i figure airspace? i havnt figured out what subs to use yet but that will be anothr topic lol. thanks in advance to everyone

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Pioneer DEH-400UB
Kicker 150.2 mids/highs amp
MB Quart fsa 216 front stage
AudioPipe gd-6001 mono amp
AUDIOQUE SD 15S PREFAB BOX!
C&D Technologies Battery, 1500 CCA, 134 amp storage
149.5 DB's

Are you building an SPL car or do you want it for continuous musical use? Either way, you need to isolate the air between the front and the rear of the sub for max efficiency.
If you're just doing the trunk, then MDF as large a surface as possible. Then you need to inject expansion foam into all crevices and openings that the wood doesn't cover. This walled method is called free-air and is quite common with decent results.
The cargo volume is between 11.8 and 12.8 for the 2-door and 4-door respectively.
If you're walling directly behind the driver's seat then I suggest you check out similar SPL vehicles like THIS or dB Drag

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Prepare your future. It wasn't the lack of stones that killed the stone age.

well i dont have a hatch so i think ill just flush mount a thick piece of mdf right behind the seats. would i need any other wood on the bottom or top or will the car act as a sturdy enough box? otherwise i would build a huge box and put it in the back, but it wont fit :(

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Pioneer DEH-400UB
Kicker 150.2 mids/highs amp
MB Quart fsa 216 front stage
AudioPipe gd-6001 mono amp
AUDIOQUE SD 15S PREFAB BOX!
C&D Technologies Battery, 1500 CCA, 134 amp storage
149.5 DB's

You can always build a big box assembled inside the car piece by piece. The OEM tin is never good enough as a box.

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Prepare your future. It wasn't the lack of stones that killed the stone age.

but i dont really want a box that  i will have to tear apart if i sell the car. so free air behind the wall is a no go? then i can just slant a big piece of wood down to the opening behind the back seats and seal it partially in the trunk. i would show some designs i would like to try but i can never find a way to do it on this website. if anyone has designs that they would like to share that would be awesome

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Pioneer DEH-400UB
Kicker 150.2 mids/highs amp
MB Quart fsa 216 front stage
AudioPipe gd-6001 mono amp
AUDIOQUE SD 15S PREFAB BOX!
C&D Technologies Battery, 1500 CCA, 134 amp storage
149.5 DB's

As a rule of thumb (who's thumb I don't know) a bad box is better than a bad wall.
Don't forget that that a DEI D1200 is 400 watts x 1 chan. @ 2 ohms /
600 watts x 1 chan. @ 1 ohm.
I'm not sure what subs/box you had but if it's the Kickers in your sig then refer to the recommended enclosures by Kicker.
It's also possible that you got used to the volume and now it's time to upgrade.

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Prepare your future. It wasn't the lack of stones that killed the stone age.

yea thats exactly right. i had 4 cvrs before these and these just arent loud enough lol. but yea i could do about 6 cvr 10s. or 8 comp 12s. or like 4 L7s. but yea that amp is gonna get upgraded as well

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Pioneer DEH-400UB
Kicker 150.2 mids/highs amp
MB Quart fsa 216 front stage
AudioPipe gd-6001 mono amp
AUDIOQUE SD 15S PREFAB BOX!
C&D Technologies Battery, 1500 CCA, 134 amp storage
149.5 DB's

I have 3 words to describe what you are wanting to do.  Rattle Rattle Rattle.   You are not going to like the sound of it.

yea im going for SPL more than anything. lol thanks 4 the heads up.

-------------
Pioneer DEH-400UB
Kicker 150.2 mids/highs amp
MB Quart fsa 216 front stage
AudioPipe gd-6001 mono amp
AUDIOQUE SD 15S PREFAB BOX!
C&D Technologies Battery, 1500 CCA, 134 amp storage
149.5 DB's