- from photo store, the mount is a 5/8" slide on to 1/4" - about $13.00
- the little adapter is also from a photo store 1/4" to 3/8" adapter - about $2.00 for pack of two
- the big adapter came with a mic mount, but you can also buy them from several places.
the 5/8" adapter that I have has inner threads of 3/8" on one side and inner threads of 1/2" on the other. the tiny piece can be inserted into the 1/2" side and screwed into the 3/8" threads for a tight secure fit. screw that assembly onto the 1/4" thread by inserting the 1/4" thread into the 1/2" side until it mates with the 3/8">1/4" thread. you should have the small piece at the "top" and use tension of the 5/8" piece against the adapter to keep everything stable. If you do it the other way, you run the (admittedly small) risk of lateral torque snapping the little coupler. In that picture of the assembled mount adapter, you can sort of see where the little adapter is visible between the outer threaded piece and the 1/4" threads.
If you can find a single part 1/4" to 5/8", I'd use that. But as pictured, the mount is sturdy as hell.