4 cu ft looks about ideal for this driver, although 3 cu ft ain't bad either. Don't sweat a little box volume.
The secret to dowel bracing is expanding glue. You cut the dowels before box assembly, but install them after. Sand the ends judiciously to get a good fit. Urethane glue (Gorilla glue) and PL Premium construction adhesive, among others, expand as they dry. You get a stiff foam where it squeezes out, and a good bond where it doesn't.
- Cut the dowels a little short, like 1/2 mm
- bevel one end a little on one side so it it fits between the panels. The bevel also forms a pocket to trap glue
- apply a bead of glue where you want the dowels
- stick one end in the glue pile
- swing the other end into place, trapping the glue under the bevel as you swing to perpendicular
- let dry completely
This type of brace works by connecting two sides, so they can't move out of phase. Since that's the only mode the speaker can drive, no resonance can occur. Think in terms of a balloon; add air and sides move away from each other - 180 degrees out of phase. Here's a god pattern.
Flush mounting a sub is optional, as are grills. One thing to consider is a double-thickness baffle. This side has a big hole in it, so extra support won't hurt.
For finish, consider truck bed liner. No priming, no sanding, looks really good when dry. There's also roll-on material with similar properties.
Get building, and...