There are 8 perimeter screws. Inside the perimeter there is a thin and maybe 3/8" wide strip of foam sealer.
That's it.
Unless someone had them apart at some point and then caulked (which should be able to be broken free) or glued (which would be tougher to break free) the crossover panel should be removable.
Only guessing but the crossover panel was cut from the back panel itself so the actual width of the opening would be whatever the saw blade width was.
I still wonder if humidity has caused swelling to the cabinet and/or panel.
It does sound as though taking out the crossover panel screws AND top woofer (I found the woofers to be lighter than I expected, but you don't want to let it just fall out) and then using some persuasion (rubber mallet and something (like a pipe or a long slender piece of wood) to press against the panel from the inside, make sure to put rags or something on the end of whatever you use so it doesn't gouge the crossover panel) to break it free.
^That procedure might be best attempted with a helper. One in front with the mallet and the other in back to make sure the panel does not just fall out once broken free.
Maybe it is a sign.
I was going to do relatively minor maintenance on my car a few weeks ago, drain and flush the coolant.
Was all set to do it and a voice in my head said "you're going to break the drain plug" which would leave me with no car.
Decided to put it off until better (warmer) weather, or let the repair shop do it before next Winter.