I haven't filmed from a saddle so take this for what's its worth, I want my cam hand high, I don't want to reach for it. Need to be able to easily reach when seated or standing ready for a shot which basically means hip high as you mention. I used to have my arm mounted to the right hand corner of my climber if that helps at all. As a right handed shooter with a bow my left hand is busy holding bow so that leaves right hand to operate cam.
Post up pics of your arm after you fab it!
Post up pics of your arm after you fab it!