Get yourself a pack of those white foam air diffusers
. Now take the double-ended plastic barb that you use to connect the diffuser to the airline and drill a hole in the top of the Magnum's intake strainer so that one side of the barb fits snugly into the hole. Connect the tubing from your CO2 bottle to the other end of the barb.
Now between the CO2 bottle and the Magnum you should probably install a check-valve to prevent back pressure from the filter forcing water into your CO2 bottle. Installing some form of bubble counter/scum catcher inline would probably be a good idea, too.