Hi Arie,
That is concerning, as that's a significant gap that could affect the evenness of the diaphragm charge in the areas where it's loose. If it's just due to weakened bonding glues vs. a warp, then what I'd suggest is creating a new frame for the panels that incorporates top and bottom supports that clamp the frame together across the horizontal dimension.
That means those structures need to have curved slots and a design that allows for adjusting the front to back tension on that curve.
It is one of the design weaknesses of the older large panel speakers to not have that top and bottom edge clamped, my Monoliths have a similar open top, and I've been toying with the idea of a new metal-based frame that would allow for adjustable and damped clamping forces in both vertical and horizontal planes. That would mitigate problems like you are reporting as well as further dampen any panel resonances when playing loud mid-bass notes.
With today's CAD-driven low volume or one-off manufacturing options, this should be doable if you know or engage someone with Spiceworks or such skills.