I needed to do something similar recently, drug out my collection of flash brackets tripod head parts, etc. Finally settled on taking an old pan head, setting it at a right angle, then replaced the handle with a bolt so it wasn't in the way. It was late night at the time, so a hardware store run wasn't an option. B&H has a number of right-angle brackets that fit Arca-Swiss quick release mounts, adapting one so that it could be fastened to a Manfrotto hex plate might be possible. Or else, Manfrotto does have an Arca type QR mount.
I have two Manfrotto heads with hex plates, another Cambo head that has their own QR style, I am seriously considering switching them all to Arca mounts because compatible plates and adaptors are so common for those.