You have a couple of options - 

1. You can simply use some vinyl tubing with an outer diameter of 10mm.
2. Thread this into the hole of the bung this will allow a good enough seal to create the blow-off tube by submersing the other end of the tube in a container of water or sanitiser

Alternatively - 

1. You will need a 1.5" triclamp to hose barb. 
2. This can be tri-clamped to the lid of the fermenter and a vinyl hose can then be attached and submerged in a container of water or sanitiser to create the blow-off tube.