How about a switch from an e series surefire. They are small and can handle quite a bit of current. I know I run a awcr123 in mine and so far no problems.
Do you need to add a switch? The twist on/off to make /break contact is a perfect switch, nothing to go wrong, low resistance path.
Are you thinking to cut down to 1AA size? 
A twist on/momentary on switch like the e series would be a nice feature and not too difficult to make but a simple twist on tail cap would be the easiest solution.
If i did cut it down to a single AA (14500) then that would allow me the freedom to choose the thread pattern to get the best twisty performance.
If i didn't cut it down and found a 3A/8.4V + driver then i would just remove the spring and install a brass puck into the original end cap.