I've done two things to add lashing points to the interior of my D1. First, I bolted in a pair of D-ring lashing points to the cargo space floor just inboard of the storage cubbies. I drilled through the floor (after checking what was underneath) and backed the hardware with big fender washers to spread out the force and rubber washers to keep out moisture. Each D-ring has two bolts and two washers.
Second, when I remove the rear seats for extra cargo capacity, I occasionally use the seat mounting holes for single-bolt D-ring lash points.
I don't treat either of these approaches as being heavy duty. Their purpose is to keep stuff still and quiet. This system is not intended to secure cases of tools and spares in a rollover.