Watching the Mercury transit with improvised devices
Mercury's angular diameter on transit day will be 12 arcseconds.
A camera obscura using a 12 mm aperture could resolve it; one lens from +0.75 diopter reading glasses, if you can get them, will project a bright 12 mm image of the Sun at a distance of 1.33 m.
Note that a larger aperture or a shorter focal length will make the Sun image hotter than direct sunlight unless you add a filter.

Test with sunspots before relying on it for Mercury.
If it counts as improvised, projection with 7x35 binoculars easily showed me the 2012 transit of Venus.

Update: I tried both of the above with the Nov 2019 Mercury transit.
With handheld wobbling, neither showed it.
Someone else's 50mm binoculars did, probably because they were mounted on a tripod.