Apart from building the circuit on a breadboard, I have neglected the physical form factor so far.

I now whipped up a circular PCB with a diameter of 20mm. It uses a CR1220 lithium cell which should be not much harder to obtain than a CR2032.

KiCad files and production data are on Github.

If you want to build this, beware that I haven't tried the particular MOSFET and inductor in the BOM. The LED may also not be the absolute best choice, but it's what was in stock at LCSC.

This could definitely be made smaller, if one wanted:

• use smaller passives, 0805 is what I have as an E6 kit and it's easy to solder by hand.
• use a dual BJT transistor package (MMDT3946) for Q1/Q2, but then Q4 is the odd one out.
• use a smaller package for the monostable. Nexperia has a SOT1116 which is 1.2 mm x 1 mm with 0.3mm pitch.
• even smaller, even less popular battery size.

But at this point I don't really have a concrete need for a particular form factor, so I haven't ordered PCBs yet.