This looks like my direct drive MT-G2 mod,but with 26650 cells. I killed the switch in that one (although I did it by shorting the batteries via the reflector). I temporally made it a tailtwisty because the threads were anodised (but it looks like the tail threads in your light are not anodised). But recently I installed the spare switch that came with the Shadow and that one keeps working, at 6A (I use IMR cells in it), although not for more than two minutes at a time thusfar (the burn-my-hands-limit 
). That Shadow switch is tiny, smaller than yours, so the size apparently is not the important factor, but the quality. Perhaps try one (or more 
) of those Omten switches, no idea if they are really better, but at least they have a brand name on them.
Apart from the famous Mc-clicky there is just no variety of high amp switches around unfortunately 
.