Drill out the Tee fitting where the bearing goes, to a depth of approximately 9/16" with a 17/64" drill bit or use a 1/4" bit and finish with a dremel tool (carefully you just want to make sure the bearing doesn't stick) The actual depth your looking for is just deep enough for the bearing to block a bb from passing from the magazine (it doesn't take much, if you can see a little of the bearing that should do it).

Only use 1 Ball Bearing instead of 2. In place of the second Bearing cut a spacer out of 1/4" copper tubing approximately 3/8" long, finsh sizing the spacer by filing it down so when assembeled the bearing sits just low enough to pass a BB from the magazine.