I used a footprint compatible to Tag-Connect to program the SAMD21. This is only one time needed to burn the boot loader.

You could use the original our use a DIY attempt. There is alredy a DIY solution by Timon which you could find here: https://github.com/timonsku/oshconnect

This was partly my inspiration for my printed Adapter. I use the same Chinese pogo pins. I was just not happy about having an extra PCB there. The white part should be printable on most hobby SLA printers, although I used it to test JLCPCBs new printing service.

To this it is possible to older some cables and a little 10p SWD break outboard (thisor similar)

With this little board it easy to connect to your Programmer. I used an Atmel ICE for flashing the bootloader from the bigger Metro M0 Express.