My 2200 is a towlite. The shelf below the front window came out easily by removing 10 - 12 screws. Removing the shelf allows better (but not ideal) inspection of the front to side joints.

Photo 1
The roughly 1x3 plywood beam is the strength member. That gap in the wooden box channel you referenced was an open conduit used to run wires as needed. Also notice in photo 1 that the front wooden beam is not aligned with the side beam. The front beam is 1-1/2 inch lower.

Photo 2:
In photo 2 of the long side, the curve ends where the lift cable is attached. Where the green & white wire curve down at the front of side wall, a rusted broken screw can be seen. That screw fastened the steel & wood. Both the sides & front had 'laminated beams' using both steel & wood. The weight of the wall & top is what curved the steel. When the top is down for travel, the weight is now carried by your hockey pucks and the curve goes away. This curve can be seen on the inside by looking at the top half bottom lip to bottom half top lip alignment. The alignment would be better where at the lift cable attach point and get worse closer the the front.

Photo 3:
It has been a while but I think that front steel tube structure was fastened to the side steel tubes with sheet metal screws. The screws & surrounding metal had rusted & the connection failed.

I hope you can get that below the shelf out so you can inspect the area better.

Well this thread was an unexpected godsend, restoring a 18' towlite and have the exact same issue. Good to know my "fix" idea has been tried by someone else and worked lol.

There are more photos available if you have a need. Post photos of your project if you want any comments.

Will do! It'll be a few days once the snow melts off and weather stabilizes so we can get it opened up a bit more.