While water-only humidity trays are pretty much useless, saturated salt trays are not! The rH over a box of kosher salt and water in a closed container is 75% over a pretty consistent temperature range. https://www.engineeringtoolbox.com/salt-humidity-d_1887.html (Photographers use this trick to keep paper moist for hand-applied emulsions.) Other salts can get the rH even higher, but salt is easy to come by. And because the salt concentration is insanely high, nothing will ever grow in it, so you can pretty much use the same tray forever.
While water-only humidity trays are pretty much useless, saturated salt trays are not! The rH over a box of kosher salt and water in a closed container is 75% over a pretty consistent temperature range. https://www.engineeringtoolbox.com/salt-humidity-d_1887.html (Photographers use this trick to keep paper moist for hand-applied emulsions.) Other salts can get the rH even higher, but salt is easy to come by. And because the salt concentration is insanely high, nothing will ever grow in it, so you can pretty much use the same tray forever.