Garmin HRM on Polar strap
Garmin heart rate monitor straps are very high quality, but quite expensive. Polar HRM straps (Polar SoftStrap) are a more modestly priced alternative. They are compatible, but have to be modded a little bit.
This is about the HRM-Run; the “classic” design: pod and strap are sold separately.
There are raised edges around the connectors on the Polar strap (arrows). They need to be trimmed down, or the Garmin HRM pod will not fit quite right, and pop out of it the minute you start running.
Cutting the tough material (it’s probably silicone) is extremely hard. A hobby knife can just barely cut it. But a pair of scissors will work well to chop off little pieces one after another.
I used a hobby knife to make eight 2mm incisions in the rings of silicone, then the scissors to trim the pieces between the incisions. The result may not be cosmetically perfect, but you don’t see that anyway, and it works very well.
During ten runs with this combination there were no signal dropouts, and the strap is easier to adjust than the sophisticated Garmin three-loop system.