Yeah tabs. Here’s the bottom:
The support towers were placed so they wouldn’t block any of the holes in the top. There’s no room for error but as it happens the Glowforge is stupid-accurate, so it fit perfectly.
Support towers are necessary so the vacuum doesn’t collapse the box, and to just give you a solid base on which to push down while sanding.
Mid assembly:
And the port where the vacuum plugs in (where the original picture was taken):
No glue on any of it, tight tabs and pegs. Very sturdy. Non pg 1/8th MDF.