Build guide — sealed case to boombox

1. Measure before you choose

Put a tape in the case you own. Interior length, width and depth; then the depth of the lid and the depth of the base separately. Subtract the foam, if you are keeping any. Write the numbers on the case with a paint pen — you will look at them a hundred times.

Baffle rectangle = interior length × interior width, minus a border for the wall radius and latch ribs. That rectangle is your maximum cutout area, and it does not grow.
Depth behind the baffle = interior depth minus whatever the amp, pack and wiring occupy. Two inches is a realistic reserve for a 2.1 board and a pack.
Which half? The base is deeper than the lid on every case in this library. Drivers that need depth go in the base; the amp and pack usually live in the lid.

If you mount on both faces, the depth budget is shared. A 6″ deep driver front and a 5″ deep sub back will not fit a 9″ interior once you allow for the board between them.

2. Lay the baffle out on paper

Full-size, on cardboard, is better than a screen. Circle centers, flange diameters, bolt circle, terminal clearance and the path of the latch hardware.

Draw the flange, not the cutout. The flange is what has to fit side by side.
Keep at least 0.75″ from the flange edge to the case wall if you are bolting through the shell, more where a latch or hinge boss sits.
Mark the cutout center with an awl, then drill a 3/8″ pilot. A hole saw follows the pilot, not your eye.
Check the driver's terminal position. It is the part that fouls a brace or a wall.

3. Cut the shell

Molded polypropylene (Apache) melts and grabs. Pelican's Copolymer is more forgiving but still wants a slow, steady feed. Either way: mask, support, cut from the outside with a backing block, and clear the chips constantly.

Hole saw for round cutouts. Run it slow, let the teeth cut, and stop if the plastic starts to smear. A smeared cut will not seal.
Router + circle jig for a cleaner edge and an exact diameter. Best result, more setup.
Jigsaw for rectilinear face-plate openings, with a fine-tooth plastic blade and tape over the cut line.
Chamfer the inside edge lightly and deburr both faces. A sharp edge cuts the gasket.

The case is no longer waterproof. Every cut voids the rating. Gaskets and a vent make it splash-resistant. Nothing here makes it submersible again, and no build should claim it.

4. Seal and fasten

A driver bolted to a thin shell with wood screws will leak and eventually crack the plastic. Use a backing ring or T-nuts, and gasket both sides of the joint.

Closed-cell foam or butyl gasket tape on the flange. Not silicone caulk — you will want to remove the driver one day.
Through-bolt with T-nuts or hurricane nuts into a 1/2″ plywood or printed backing ring. Washers both sides, nylock or threadlocker.
Seal the bolt holes with a dab of non-hardening sealant. Air leaks whistle and cost bass.
Check with a leak test before final assembly: press the cone gently and listen at every joint. A $15 stethoscope or a rolled-up magazine works.

5. The printed face plate path

A plate lets you run three or four small drivers, or one large one, on a case whose shell you do not want to drill more than once. It also covers a bad cut.

Material: PETG or ASA, not PLA. A closed case in the sun will soften PLA.
Wall: 4–6 mm, with ribs between driver openings. A flat plate with four 4″ holes in it is a drumhead, not a baffle.
Gasket both sides: plate-to-case and driver-to-plate. The plate is a second leak path.
Fasten to the case through the original foam-screw holes or through-bolts at the corners, not to the thin lip.
Recovered area: a plate spans the opening, so the usable border drops from 0.75″ to about 0.30″. That is where the extra driver comes from.

6. Power and protection

Fuse at the pack positive. Size to the wire, not the amp: 10 A for 18 AWG, 20 A for 16 AWG, 30 A for 14 AWG.
Master switch rated for the full current, on the positive feed, accessible without opening the case.
Pre-charge the amp input capacitance through a 10 Ω resistor, or use an anti-spark connector.
Star ground at the pack negative. One point, not a daisy chain, or you will chase hum forever.
Strain relief every wire that crosses a hinge or a lid seam. Silicone wire, not solid core.

Voltage check on the bench. Set the pack to its nominal voltage, not full charge, when you first power the board — a bench supply with a current limit saves the amp if something is wired backwards.

7. Heat

Class-D is 88–92% efficient, which means a 100 W board makes 8–12 W of heat inside a sealed plastic box. That is fine for a while and not fine at a beach party.

Bolt the board to an aluminum plate and bolt the plate to the case. The shell is a poor heatsink but a large one.
Keep the pack away from the heatsink. Li-ion at 45 °C ages fast.
Vent with a Gore-style pressure vent if you run high power. It passes air slowly and keeps splashes out.
Test at full tilt for 30 minutes before you close the lid for good. Feel the plate with the back of your hand.

8. Tune it, then finish it

Sealed is the simplest and the most forgiving in a small case. Expect a higher F3 than the driver's spec sheet suggests.
Passive radiator extends low frequency without a port. Add mass to lower tuning; measure rather than guess if you can.
Stuffing (polyfill, 0.5–1 lb/ft³) makes the box behave larger and kills internal reflections. Do not block the driver's rear vent.
Measure the impedance sweep if you have a DATS or a soundcard rig. Two peaks close together mean your tuning is right; one peak means it is not.
Finish with bedliner or vinyl, label the inputs, and write the build date inside the lid.