you dont HAVE to use liquid nails, regular elmers wood glue works great. however...make sure you have a way to secure the bottom panel (which you said is already in the car) to the rest of the box (e.i. screws). if you cant do that, then build the box with another bottom piece which can be secured to the existing panel. i like to run a bead of silicone (any kind will do) around every seam on the inside of the box. for the top baffle, you may have to use your finger to spread it around. if the subs are hefty...make sure you brace the box well. a couple 2x4s between the top baffle (where the subs are mounted) and the back baffle is perfect.