[0zelot]

this is possible! you can use a thin foil made from e.g. mylar, for example taken from a cracked-open old large foil capacitor and superglue one layer on the rim of the diode's metal case. It can then point through a hole into the outside of the tin box if not held into direct sunlight (some additional foam ring can be used to help block more light). Using one of the sturdy aluminum die-cast cases I've listed in the schematic should be favored in a mobile scenario. Much less affected by vibrations/microphonic effect.

[supernova87a]

Interesting, thanks!