Done with M4x15 bolts and their corresponding nuts
The M4 rounded head is 7,2X4mm. The nut is 7,2mm wide
You need to order a Garmin support like the one found in Aliexpress https://s.click.aliexpress.com/e/_Dk3z8vl. You hold this support using the small bolts provided
This suits a <42mm wide belt with a thickness around <5mm
No support is required. PLA, ABS or PETG can be used as your choice