After spending way too much time looking for an existing solution, I ended up creating my own desiccant holder that supports the rectangular style hydrometer that can be stored in a spool. I printed this with 0.4 nozzle, 0.2mm layer, with paint on tree supports for the hydrometer surface (bottom). I printed this with CoPE because I was trying to get rid of this god-awful filament, but should work just fine with PLA. Container has a diameter of 54mm and height of 61mm. Might not fit in some cardboard or smaller spools.