An emergency fix could be some wire coat hangers to create a "frame" or "cage" and some duct tape.
Cut the coat hanger into pieces and bend ends to fit over opening. Then use duct tape to secure wire frame and enclose the opening.
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs