A chest freezer - gutted out.
Although it may be possible to do this with some other type of container, the insulation of the freezer would definitely help speed the process. You can pull anything not needed out of the freezer, especially if they add weight. Note: Be a responsible citizen and make sure the freon is evacuated from the system by a qualified shop!
A solar water heater.
There are several instructables on making one of these, so I won't get into it. Check these out:
Because this device will be used out in freezing weather, using anti-freeze would be a good idea.
A heating element.
One could be made the same way as the lines in the collector, or parts may be able to be scavenged from the freezer. Check out if the evaporator or condenser might do the trick.
A protective grate.
This could be made of a heavy gauge expanded metal, or rods about the thickness used in large BBQ grills. Sitting right above the heating element, this will bear the weight of the snow and keep the heating element from getting damaged.
Plumbing from the collector to the heating element.
You may want to consider hoses that are well insulated to conserve the heat. Not having played with this idea yet, I'm not sure if the convection of the heating liquid would be enough for flow control. Check valves may be considered. This would also be a good place to install a filling point for the heating liquid.
A ball valve or garden faucet would work, plus some hoses to run to your drain or storage tank.
Wheels or casters.
This will be rolled around in the snow, in the yard, in the driveway, so decent sized wheels would be a good idea.
It would be a good idea to have hinges attaching the panel to the door of the freezer so it can be lowered when not in use, and can have the angle adjusted for the best efficiency. See http://www.freesolarpro.com/weblog/31
on how to properly aim your panel. With the correct angle calculated, you can have props on each side of the panel to hold it at the that angle.
If anyone is able to get one of these built and working, let me know :)