I bought an old Bosch sander from a garage sale which came with a filter but no adapter to use it, so I modeled one quickly in MS 3D Builder.

Leaving the files for both:

• The entire adapter which in the tool side has a 26mm internal diameter port
• Just the female part for the filter side in case you want to modify it for a larger tool port.
• Just the male part for the tool side, it's just a cilinder tapered from 26 to 25mm