While building our RV-10, we wanted a secondary door latch. We were concerned about possible rescue efforts in case of an accident and complexity and cost.
Here's what we did and so far it works great! A machinist friend, Matt Banks <perfwingsnwheels@yahoo.com> made the parts and made them adjustable to allow for slight variations in doors. (Matt has a beautiful Fairchild 24 with a Jacobs engine.)
The receiver end has three screws that match the pattern of number -8 screws already there in the fuselage . The door handle needed a simple mod with a doubler plate inside the handle to add strength. The end of the secondary latch arm has a machine grip in it.

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Vern Darley
RV-6A N680V / RV-10QB N353RV
Luscombe 8E
Falcon RV Squadron
KFFC Hanger D-30
Peachtree City, GA
www.falconrv.com
770 310-7169
EAA Technical Counselor #5142
EAA Flight Advisor #486336
ATP/CFI/A&P/DAR