Old school idea here which would be a bit of work but might help -

Get 3 - 4" wide fiberglass cloth and wrap the center section - this is fairly common practice in kit building. The cloth is attached to the wing using thinned epoxy or finishing resin. I agree that a "no gap" situation is better, but the glass cloth treatment would add strength. Would have to strip the covering around the center section and recover but a non-folding wing is ideal for a trainer...