1 Check Fat Daddy's website too - I think on his website you can get an extended fire button screw and 510 extended replacement screw for the same price as that ebay listing for just 1 screw.
2 Have you tried different atty's on num-num w/ flat top batts. I wonder if the atty has a short 510 screw so you have to turn num-nums 510 screw in to contact it. So then you're flat top batt is contacting num-num's 510 screw when pressing the fire button.
3 Or how about this to turn your flat tops into button tops - just $2.50
Flat Top Battery Adapter
4 Or for <$1 try one of these on top of num-nums 510. The you could maybe unscrew num-num's 510 screw to get it closer to the flat surface of the batt when the fire button is pressed.
Circuit Board Spacer