[morzh]

First, I cut, starched and ironed the cloth.
As I said before, I have the second board so the unwieldy one with the speaker already mount wasn't necessary to use.
Then, as Mike suggested, I used a cardboard box as the holder. The box is sturdy enough to not collapse under even more considerable pressure than the one I had to apply (not much at all).

   

Then protected the cone.
   

Then glued the top, dried, and then the rest of it, having stretched while gluing.

   

Then installed in the cabinet.

   

And the chassis.