Judging from the picture, the light box is not that big! A large part of the reason for this situation is the quality of the board! Acrylic panels have very good thermoplasticity and will not deform so much even if they are exposed to the sun outdoors! In view of this, the processing method is: take off the board, heat it in the oven of the blister machine, and then flatten it on a flat plate.

Then when installing, you can put a few transparent acrylic strips in the middle of the light box to support it! The above is for reference only!