Coospo Heart Rate Monitor Chest Strap.

I've tried mine with Garmin, Polar, Wahoo, it will work with any device that accepts a HR on bluetooth or ANT+ (bike computers, smart watches, gym equipment)

So much cheaper than other big brands and does exactly the same.  I've used mine for 7 years and it's still going strong, just needs the occasional replacement CR2032 battery, lasts ages (300 hours).

48 grams

10mm thick

Wide range of strap adjustment

IP67 water resistance keeping sweat and water out on rainy days