I’m thinking of an easel with a way to screw into the back of the canvas so it hovers in place giving access to all sides.

Or maybe a reverse clamp that pushes against the inner edges of the back of the canvas that connects to an easel.

The problem is that when I paint the bottom of a canvas I need to wait for the top to dry and then flip it. Or I need to hang the canvas, but it isn’t held solidly in place.