I am running a hammer right now in fact.

I am using the flat chi you top cap from FT and I have switched the place of the battery extension tube and the switch. I have replaced the M3 screw that came in the switch with a longer M3 brass screw, I silver plated it, but that's not necessary if you keep your contacts clean. You don't need to track down a M3 screw of exactly the right length, just cut and file a longer one. Look for a big thick head on the screw to get more contact area and durability. A lot of places sell them. You want to size the screw so that the screw comes into contact with the inner brass tube at the same time the part of the switch you press comes into contact with the locking ring section. You want the switch screw to be screwed all the way in so it's not loose.
I am still considering using the original top cap with a custom solid copper pin I hammered out of a piece of 101 copper rod, but I find the original top cap is a bit "grippy" and has gotten some of my atties stuck. I may try polishing it to a near mirror finish with some red rouge to see if that makes it non-sticky, but the chi you flat cap works just fine out of the box. I am using 18650 so I had to remove the second pin in the chi you cap, Or you can use the kick tube and another longer M3 screw to replace the second screw in the chi cap if you want to use a telescoping screw. There are several possible configurations for the hammer. Nothing that can't be done with a piece of sandpaper and some very inexpensive brass hardware screws.
Using these "custom" parts, I have never had a short circuit in my hammer mod. Or my other hammer mod. Just make sure everything is making good contact before you fire 'er up.