The idea was to have a leaf spring that is held down at either end, achieved by tightening down on a knurled brass knob at either end. The reason I wanted to do this was to allow those people who may have difficulty in getting the normal leaf springs in position. My way is slower for sure, but I'm hoping easier for those who might have some issues with manual dexterity.
As for the leaf spring bowing up in the middle... it will still be screwed into the back so will not rise up in the middle. All will become clear when I get to building that bit! The brass knobs are only resting on the spring in that image, they will not be fixed to it; instead, the leaf spring will have a slot cut into it at either end to allow the threaded portion of the knob to fit through.
Here's a quick drawing of the cross-section that I hope will make it clearer...