Use #4-40 3/8 flat Phillips machine screws through the enclosure and mounting holes on the circuit boards to secure them inside, aligning the ports with the cutouts on the enclosure.
The enclosure has countersunk screw holes so the screws sit flush to the plastic surface.
Apply a piece of foam double-stick tape to the battery and stick it to the inside of the enclosure lid, within the rectangular outline.
Carefully plug the battery into the BATT port on the lipoly battery charger. You may need to use tweezers or pliers to get it into the small space.
The above pictured version of this circuit uses a double-ended JST cable instead of one soldered directly to the LOAD pads on the PCB. There is no reason to do this unless you like practicing your wire splicing and have extra JST wires hanging around your desk (I'm guilty as charged!).
Place the lid and secure it with four more machine screws.
If the solar panel is plugged in and in bright sun, the battery will be charging. You can toggle the PowerBoost power with the slide switch, and you can peek inside the USB cutout to see the green light indicating power is on.