Beseler used to make a easel for borderless prints. It works pretty well. It grips two edges of the paper with slightly canted metal pieces that are shaped to not interfere with the light (much). These provide just enough of a grip to hold the paper flat. They also provide consistent positioning.
A DIY approach would be cut some sheets of laminate to the same sizes as the paper you will use and paint them flat black. 3M makes a removable double coated tape (667) that you cab put on the boards to hold the paper.
A DIY approach would be cut some sheets of laminate to the same sizes as the paper you will use and paint them flat black. 3M makes a removable double coated tape (667) that you cab put on the boards to hold the paper.