I put the #s thru WINIsd, and at that small a box size, you'll need to use 2 4" vents about 13.1" long. This will get your port velocity down to .06mach, which should be OK....

Another option, if it will fit in your vehicle, is that 6cuft tuned to 30hz box on the spec sheet. Using 3/4" MDF with outside dims of 44"W X 17"H X 18.25"D, with a single slot port 15.5H X 2"W X 7.5" D will tune it to 30hz....

Easy to build, and it should pound pretty good.....

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BigRedGuy
Still bumpin' after all these years
Kenwood/AudioControl/Orion/JL Audio/Lightning Audio