Lookin' Good!
I had an idea for you ... I don't know if you solved your issue with the arms folding in, but was thinking you could drill a hole down through the triangular body top, arm, and down through the bottom body. Then, take a 16 penny nail (or nail of your choice), cut it to length and pin the arms in place. You could then put another couple of holes through the body which could be used to pin the arms in place for shipment. I have seen nails which have an extended head on them which would work well, or you could even use a small bolt cut to the right length (or purchased at the right length) if you wanted to. It would just keep your flight arms from moving when you didn't want them moving was my thinking. Looking again at the first video, you could even drill a hole down next to where the arm sits at its extended position and it would hold it tight during flight.