Here's mine that I knocked up after trying to use solar bags to heat up the water for the shower pump in the damp cold 2007 Le Mans 24. It heats up the water ok but if you want it very hot then just start it up with a bigger bucket than the one in the video and pop the handset in the bucket for a while to heat up and recirculate then hey presto nice hot shower. I just coiled the copper pipe a round a paint tin to get the shape and by putting the alluminium plate on the top keeps the heat in. I might try wrapping the pipe coils in ally as well to see if it will heat up better, saving heating it twice or I may be able to get a smaller coil inside.
Cheers Ken
http://s874.photobucket.com/albums/ab301/kenf27/