an update: That spray held precisely less than a day and started leaking again (a bit slowly)
I got a quote of $300 and 2-3 hours of work to replace the collector box cover.
My colleague recommended me his HVAC guy - called him - he said he'd do it for $150, and he finished in about 1/2 hour.
Now four days later, no leaks.
I've bought ZigBee smart water sensors on amazon, configured my Home assistant (open-source smart home hub software) to send me alerts in case of water leaks, and placed sensors in a few critical locations.