Stress a motor mount places on rear crossbar

I am concerned about the stress that all the motor mount designs I've seen put on the rear crossbar . I've decided on the motor mount design by KeithB posted here viewtopic.php?f=23&t=57851 . I am considering adding an additional crossbar aft of the crossbar between the hulls that the motor mount arm would bolt to . The crossbar would just rest on the hulls in some fashion without actually being bolted to them to allow for flexing. What are opinions about this. The outboard I have weighs 37 lbs

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2016 Hobie Getaway
1978 Nacra 5.2