I didn't make pictures while assembling the battery pack holders, but it didn't take more than clamping the Makita 644808-8 terminal between the two parts of the 3D printed parts, then screwing it tight with the DIN912 M4x40mm screws and DIN985 M4 lock nuts.

Then came sawing, drilling, and countersinking the alu composite panel:

Attaching the battery pack holders using the DIN7991 M4x16mm screws and DIN958 M4 lock nuts:

Mounting the inverter using the DIN7991 M5x16mm nuts and DIN985 M5 lock nuts:

I then cut and crimped the cables to length, keeping in mind to leave enough space to place and remove the battery packs. Use the FLDNY 5.5-250 crimp terminals to connect to the battery pack holders, and the RNB 5.5-8 crimp terminal for screwing into the inverter:

After a quick test fit, I cut holes for the outlets, mounted it, and connected it to the inverter:

And that's job done! :)