Rocket stove to head up 700L Pool

Hey everyone
I am now working on a project to build a stove to head up my pool. I have been googling it back and forward and still not sure how I wand to have it. I work in a mechanism workshop so I have good access to tools.
But now I need your help to work out a good design.
The pool is 700 L and the pump is pumping around 20l Per min.
I would like it to out of mix of black metal and stainless steel, where the stainless part is where the water will be heated in.

sizes and setup needed.

Hi Gunnar,
Personally, I didn't build such a heater, but maybe someting like this is what you have in mind?
http://donkey32.proboards.com/thread/1708/pizza-oven-pool-heater-build

Will this work to head up the water?
Blue being the water