I want it to be very reliable, which means as few points of failure as possible. The initial versions will have a very simple, inefficient circuit. I plan to use supercapacitors for the storage, for their long life. Ideally the solar panel would produce a voltage no higher than the rated voltage of the supercaps.
I'm planning to use an attiny10 to blink the LED. It can run happily down to 1.7v, and only uses a tiny amount of power.
The housing was designed to hold the electronics, potted into a cavity, and the glass window. The enclosure applies pressure to only the front and back flat faces of the window, avoiding any pressure on the sides, or any contact with the edges. A gasket seals the glass against the metal at the front. The gasket must be ductile and made of a noble metal, unreactive with the environment and the housing. Gold would be a good choice for this. Copper may work too. The enclosure will probably be made of Aluminium due to its chemical resilience.
Once the enclosure is clamped shut, using two bars and two bolts, or a vice, it will be welded closed along the seam.
The attiny10 is very tiny, only 1kB of flash and no EEPROM. The code that blinks the LED and the pattern for the blinking must share the same 1kB of space. I wrote some python code to digest a string of text and convert it into an on/off LED sequence. The code running on the microcontroller just needs to wake, read the next bit, set the LED accordingly, then sleep for 100ms.