Hi Alex, we've got an outdoor solar shower and have done for 5 years - if thats tried and tested. We're in italy too, piemonte, so should work as well for you.
It has no storage, as it was a gift from a guy who ran a solar shower rig on the UK festival scene and this was his first prototype that he used at the Big Green about 2005 or so. all it is is a coil of pipe (1" black PE, ~75m long) fixed to a wooden cross, making a 1.5m circle (gaps between pipes as he built it by screwing the pipes to the cross with pipe fixers). It has some thin polyurethane foam insulation material (sold to go under laminate flooring in Brico OK on a 20m roll) shiny metalised side up underneath the coil so that any sun that misses the top reflects back and hits the bottom, and polytunnel plastic wrapped over the top, and a snake sets up home in there over the summer. It heats up in ½hour from cold, and gets to >65°C in summer - I have trouble because of the claber rainjet PE ½" pipes going soft from heat and letting go of their fixings. The shower space is in a polythene teepee with a pallet and a mixer tap from an old caravan we had. The teepee also gets planted with cucumbers etc. that like the heat and humidity. The coil also feeds up to the semi-outdoor kitchen sink and the washing machine. It does well in summer though not enough for everything when there are guests, but not much in winter, although we have had some the last few days. The hot pipe runs are insulated with strips of the same shiny insulation wound round the ½" PE pipes approx double layer with a black PE sheet wrap over the top to protect, all fixed with duck tape. the black wrapped stuff has survived in a bramble thicket for 3 years now but where not wrapped the insulation has been chewed by rodents (as has the stuff under the coil, but it seems not to matter).
this is the simplest way to do this.
I have thought about adding storage but think that a circulation pump would be needed because a coil would not automatically gravity feed, since the hot water in the coil collects in the uphill turns of the coil and doesn't go any further. to get gravity feed to work would need a strait matrix in the collector aligned up the slope with some kind of manifold top and bottom, which gets a bit expensive if you make out of normal pipe Tee's, also the spacing is too great if done this way.
I would not use pipe clamps to do this myself, I would do the following: get a wood drill (probably an auger) to drill a series of holes in the wood to fix to, then cut along the line of holes with my bandsaw about 3/4 to the top, fix the wooden parts into shape and then coil the pipe into the 3/4 holes (they'll clip in place) then re-fix the severed strip over the top to hold all firmly.
