This spacer will adapt Thorped's WANHAO Fan duct to a Di3 v2.1 bearing block.
One edge is a bit thinner for a better fit under the belt because I have this belt clamp installed.
Installed using two M4 x 35mm button head screws with washers under the heads, inserted from the front of the printer, with spring washers and nuts on the back. I had to grind the screws down to ~32mm and use regular nuts with spring washers, instead of nylocs, so the fan would clear.
After printing, I cleaned it up and simply glued the spacer to the duct holder.