6mm wing nut grip / handle.

See also the remixed file. The main difference is using an m6 wingnut you'll need to thread the bolt into the hole first then use it to pull the wingnut into it's captured position. In addition to providing a more secure capture, the smaller hole diameter at the bottom helps provide some more resistance to the bolt when spinning it. In exchange for no more rapid spinning the nut to tighten it, hopefully the additional plastic will conform to the threads and prevent spontaneous loosening. 

My version is the one with the bolt in it in the photo, compared to the original on the right.