The “tabs” are to cover these small voids at the base.
The Frame is assembled with the attachment of the tabs pending. Approximate hole measurement is roughly 45″ x 55″x 78″ including height of the frame. This equates to approximately 4.14 cubic feet.
Next step is to drill the 2″ x 4″s to hang the base assembly into the hole and plumb the tower base before pouring concrete.