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There is an idea here from the REUK website. https://www.reuk.co.uk/DIY-Solar-Water-Heating-Prototype.htm

There are other websites with loads of designs but I have them marked on a different machine and have not brought the list of favourites with me. Amongst the options I've seen are the simple approach of direct heating a black cylinder in a wooden box with a glass panel. The cylinder is sat on insulation to reduce heat transfer.