you will need something quite bit more substantial than the piece of ply to stop cells that wide from swelling. something like 3mm plate fully welded inside the angle iron frame to give it the rigidity, even 3mm aluminium sheet would buckle across that width.
T1 Terry
A person may fail many times, they only become a failure when they blame someone else John Burrows
Those who struggle to become a leader, rarely know a clear direction forward for anyone but themselves
If you multiply the force x the 16 flat sides it doesn't take much per cell over that area the bend the ply. I have seen the cells bulge just sitting on a shelf not connected to anything and they will most definitely bulge during normal use.
T1 Terry
A person may fail many times, they only become a failure when they blame someone else John Burrows
Those who struggle to become a leader, rarely know a clear direction forward for anyone but themselves