Aluminum is not safe for electrodes.
The aluminum foil will stick permanently to plastic and it can't be removed in our experience. It will also cause pain if you apply it to the inside of the cage as it has to bend and it can't stretch... the surface won't be flat.
Just noticed you were referring to the bottom electrode. In this case there's not enough thickness to make contact with the scrotum, maybe you can get it to adhere well without wrinkles on the bottom, but it's still not safe for long term wear.
I have used conducting rubber from an E Stim kit on the outside and inside the tube . Using a countersunk head nylon screw tighened down so that it flush with rubber on the inside is very comfortable You dont need to glue either
Steel
The aluminum foil will stick permanently to plastic and it can't be removed in our experience. It will also cause pain if you apply it to the inside of the cage as it has to bend and it can't stretch... the surface won't be flat.