My cal has the same transom mount and does flex a little with the weight of the OB. You can crawl past the quarter berth and gain access to the transom for this task. I'd also tighten all the nuts for the various deck fittings while you're back there. A 1.5x1.5' square piece of marine ply backed by fender style washers might be overkill, but they'll distribute the weight of the motor under power or while hanging out of the water to reduce the flex of the glass. Mine has been operated for at least a decade with the motor hanging from the transom with no issues.
Signature picture courtesy of Cal20 Class Assn.