Hello kr3t3n,
Yes I understood what you meant by a plastic grill, but not sure how you meant you wanted to use it, or purpose it served. If your trying to replace the plastic netting with the plastic grill, your going to wind up with the same problem. The roots will still become intertwined with the plastic grill and be impossible to separate when you wanted to harvest a potato. If you try harvesting a potato and leave the cut roots attached, the dead roots will just feed pathogens and cause disease. To keep that from happening the only option would be harvest the whole plant, clean the system out and start new plants.
You will have roots all the way to the bottom for sure. You would have to build it 2-3 stories tall to keep all the roots from reaching the bottom. Roots don't rot because their wet or submerged. They rot because of suffocation. With the water/
nutrients raining down from sprinkler heads and circulating back to the reservoir, as well as fresh air getting into the root zone there will be plenty of dissolved oxygen in the water to keep the roots from suffocating.