Unglazed uninsulated tempering tanks/batch heaters sitting in the sun have high efficiency during the day up to ~125F or so, but lose efficiency rapidly at higher temps, and lose heat rapidly at night. The net efficiency is actually pretty sucky, but it's cheap. You get more out of it if your habit is to bathe/shower in the afternoon than in the AM, using the heat before losing it to the night sky.

It doen't take much of an insulated & glazed "hot box" around it to achieve flat-plate efficiency, but it takes the same amount of glazed area to bag the same amount of heat- you can't cheat the math in physics. Commercial batch heater tanks in a flat panel form factor are available, and quite inexpensive relative to more complex systems. (No pumps, no drainback no controls whatsoever beyond a tempering valve at the output of your standard HW heater.)

Any suggesions where they can be found?

If by "they" you mean batch solar heaters, in low-frost FL you might start by looking at the Harpiris SunCache:

http://www.harpiris.com/

In frostier zones something like the BTF mini might work:

http://www.btfsolar.com/12T-guide.pdf

(Stagnation temps might get a bit high on evacuated tube systems in sunny FL.)

Or google "batch solar"- there are other options out there.  Building them from scratch is a popular DIY project in some areas.