Lay the PAM8302 PCB over the standoffs on the enclosure and hold it in place while you fasten a single machine screw to the bottom of the enclosure.
Get the power and ground wires from the PAM8302 amplifier and solder them to the positive and negative pins on the PowerBoost 1000C.
Insert the thin plastic speaker into the cylindrical cavity. Press it down until it snaps into place. Tolerances should be pretty tight.
Insert the 3.5mm connector from the audio cable into the audio jack on the Raspberry Pi
Install Raspberry Pi
Lay the PCB over the standoffs with the ports facing the cutout. Line up the mounting holes on the Pi with the holes on the standoffs. Insert four #4-40 3/8 flat Phillips machine screws and fasten them into place from the top of the Pi.