What can be used to hold a foot valve off the bottom of the lake?

One option is to build a simple A-frame out of wood and anchor it to the lake bottom. Another is to run the line through a concrete building block. Keep your foot valve at least 30 cm off the bottom, and at least one metre below the lake’s surface, to account for water-level fluctuations. Position the valve so it points down: Gravity and water pressure will help it close properly.