Great question Chris. I would make the walls only as high as they have to be to keep sand out. Mine is about 1-2 taller than my pump, and then I attached gutter guard to help keep any algae or other unwanted crud out of the pump chamber.

On the other edge of the sword keeping the chamber as small as possible but really tall will allow more water volume to be kept in the sump. Also if you loose syphon on the overflow it will only pump that much water into the DT avoiding a big overflow.