Man look's nice.
what's the net volume on that box Buck? And what is the ideal port area per 18 for loud and low? working on a box like that for my next build that will be starting soon. my volume after displacements will be 23.91/ft3 and want to tune around 27-29hz. won't get very loud starting out cuz 2 of the 18" apx's have never seen power
Start off by multiplying your cubic feet by 14-16 to get your crossectional area aka port area. Then take that number divide it by your height and you see how wide the port should be.
If you dont have Winisd, just download it, its pretty simple to use.
If you dont want to do that then use this Subwoofer Enclosure Calculators, Parallel, Series, Port Length and Volume go to the slot port calculator and enter in your dimensions and area
09 Sienna Subs : 4 hdc3 18s c pillar below windowline Sub amp: ampere tfe 8.0 at half ohm.
Headunit: Pioneer 80PRS Mids/Highs amp: PPI 900.4
Front stage: 3 way active front stage with Massive Audio CKX tweets, alpine type R 6x9 midrange in doors + Crescendo PWX 10s in the kick
320 amp Singer alt. 1 group 34 under the hood 6 group31 agms in the back 560 amp hours total.