Poor diagrams, but hopefully they will show why I went for sloping footrests.

The top row shows how small feet and big feet end up when jammed into a kayak. (Whadda ya mean, they don't look like feet?). Tests with my feet and Anna's prove it! The red dots show where ideally the foot would rest on the footrest: note that, counter-intuitively, small feet actually want the footrest to be *higher up* than big feet.

Small feet are generally attached to shorter legs than large feet. So for a kayak which may be used by many people, that suggests a sliding footrest which is lower at the bow (further from the seat for longer legs) and higher further back. See the bottom diagram.

In this bottom diagram, the second advantage can then be seen. With feet in their natural position the sole is not upright, but slanted. A horizontal footrest slide means a vertical footrest: not super-comfortable. But the slanted footrest slide means a slanted footrest, and more confort. Magic. I love it when a plan comes together.