Lots of good ideas like this over at "builditsolar.com"
"Thermosiphon" means if configured correctly, the heated water will rise, causing a natural circulation in the loop without a pump.
If you were to find an old water heater tank, you should insulate the outside and set it up so it stands vertically. Build a set of vertical heat collection pipes alongside the tank. Join the tops of the collector pipes and connect them to the top of the tank, and also join the bottoms and connect them to the bottom of the tank.
If there is some air in the storage tank (for heat expansion) the collector pipes must be full of water and the upper outlet pipe must empty into it under the storage water top level for it to circulate without a pump.
It would be worthwhile to box in the heat collection pipes, and cover the front with glass, perhaps an old glass door. Of course, set the fresnel lens at whatever distance provides the best heat focus.
If you connected the collector to the tank with flexible hoses, you could make a tracking mechanism, so the collector follows the sun.
Perhaps use a fixed collector with two reflecting panels on the sides like a funnel, and have the fresnel lens track the sun. Best of luck, and have fun.