Your 1/2 inch must be refering to the inside diameter? 1/2 O.D. should go in easily. If you use pipe too small it will slip all the way into the aluminum tube and not work. I found my piece of pipe in my junk box. Don't know what it came off of.
There is standard copper tubing that is exactly 1/2 inch O.D. that should work. Remember that copper comes in soft roll and rigid. I think they are measured differently with respect to I.D. and O.D.