The genuine Suzuki tools (some still available from Suzuki, go to Alpha Sport under Special tools) with similar dimensions to Oldfart's home made job and Pokey's genuine tool have the following dimensions:
To suit TS125/TM125/RM125M&S Internal AF =12.2mm, ID = 14.1mm, OD = 17.9mm
To suit 1976, 77,78 RM250/PE250 (36mm fork leg), Internal AF = 17.4mm, ID =24.1mm, OD = 27.2mm
There is another smaller tool with an internal AF of 8.2mm, I'm assuming this would be to suit the smaller fork legs like found on the mini bikes etc.
The later PE175 and 250's models have either a 19mm or 24mm internal AF hex shape in the top of their dampener rods. Best way to make your own tool is to weld a 24mmAF or 19mmAF nut on the end of a piece of tube.
FYI AF = dimension "Across Flats"