Re: please help!
you can go with a top firing port. if you did 60sq" (8.5 x 7) it would need to be 10.75" long to be tuned to 40hz.
as for bracing, wooded dowels are my personal favorite. all you would need to de is between the front and back. the port walls may add support as well.
2006 Jeep Liberty
Adire Audio 15" Brahma running off Cadence A7hc, Clarion hu, MB Quart 6.5s and 5.25s running off old school Punch 800a4
146+db @ 38hz with 1200w and a single 15"
Lv = [(14630000 x R^2) / (Fb^2 x Vb)] - 1.463R
*Vb = vol. in cu"
**R = sq.rt. A / Pi (for square/slot port)