I am using a plastic fire extinguisher bracket to hold my 5 lb co2 cannister in my kegerator that I purchased in from Ebay several years ago. It is for a 10 lb extinguisher, but fits the co2 cannister perfectly.
This extinguisher appears to be the same as mine (center of page, model 3870v):
I did not use the mounting holes on the back, instead mounting it to a shelf with screws through the base.
Hope this helps.