This adapter attaches to the underside of a Garmin Flush Out-Front Mount and allows the use of any accessory that has a 2 tab GoPro style connector. 

In order to attach this to your Garmin mount, you will need to replace the stock machine screws/bolts with:

 (2) M3 x 0.5 machine screws (bolts) measuring 12mm in length (length of threaded shaft, not including head) Bolts longer than 12mm will interfere with the attachment of a garmin on the top of the mount. cut or file them to 12mm or add spacers under the bolt head.

To attach an accessory, you will need:

(1) M5 Bolt at least 18mm long and matching nut (flat to flat approx 9.6mm)

or

(1) 3/16" bolt ¾" long & matching nut  (flat to flat measurement approximately 3/8")

or

(1) GoPro style thumb screw with 3/16" bolt and matching nut. Roughly ¾" (18mm) of the bolt shaft must extend past the thumb screw body. You may need to file a chamfer on the thumb screw body if it does not fit the bolt head hole in the mount. Either that or embiggen the hole in the mount slightly :D

Note:

The hole in the part is deliberately undersized for the bolt and nut. You may need to drill out the hole if you use a different sized bolt. The nut can be "heat-set" by placing it in boiling water for a minute or two, then using an inserted bolt to pull or push it into place in the part. pushing the nut in with the tip of a soldering iron or the hot, empty, end of a hot glue gun will also work. Wear gloves for this process, the nut will get hot enough to burn you!

Put a drop of loctite blue on all bolts to ensure they don't rattle loose

2nd file is the pictured adapter to convert the mushroom shaped peg on the light to attach to the gopro style mount. Recommend printing it as oriented, otherwise the tabs will be very weak. The small piece is meant to slide into the groove after install on the light. It can be glued in place.