The Single Shaft mounting block is made for 2020 which is likely too big. However, making a similar component in your shop may be doable.
I could probably CAD up something quick to give you an idea.
Edit, unfortunately, it is pretty tight to try and fit a 5/8" shaft in the profile given. Especially as far as mounting holes go for interfacing with the 80/20, you won't have many threads (2 or 3) that would interface with the 80/20 via a gusset:
http://i.imgur.com/MrckoY3.png
Shrinking down to a half inch shaft allows you to have a one piece shaft collar, as well as 3 1/4-20 tapped holes for mounting to the 80/20. The screws for the shaft clamp can also go from #6-32 to #8-32
http://i.imgur.com/KN15RSu.png