I believe the captive nut on the backside may have broken loose?
Is the spinning nut next to the seat plug? If so maybe you can grab the spinning nut with a distributor wrench. Think about it ...
Even if you get the bolt and the now un-captive nut out you won't have a way to get another nut back in there and make it captive again.
You may have to perform a little surgery on the side of the box with a cutoff wheel, repair the nut issue by welding in a new one and reweld it all back together.
I don't have first hand knowledge of the convertible issue, but it is the way I've fixed other captive nuts breaking loose.