I first tried this on an older junk tuba, just using the end of the spoon handle as a finger spatula. That worked well but looked odd. So on the newer valve section pictured here I screwed the original button into the second hole at the other end of the steel strip. (That may be more trouble than it's worth, but it looks better.) I used a dremel tool to trim off the rim of that button so that it could be screwed down farther.
Since this arrangement does put a lot of stress on the valve guide I replaced the original nylon guide with a stainless steel one that I made by filing down a round-headed screw. With a loose fit it made a bit more clatter than nylon, but with a more snug fit it does not clatter at all. The nice part is that it does not wear out. (Actually, some wear has become evident in the keyway groove after 8 years. Now I have replaced the stainless steel valve guide with a longer one made of brass.)
On a 3-valve tuba I would suggest putting an extender on the 3rd valve only, since that one normally is subjected to less wear & tear. EXAMPLE: