I was meaning NiMH - I got 15A capable ones from Kevin when I ordered the light as decent C NiMH are hard to find here. Mine are Tenergy branded 5000mAh cells. It might be easier to get good Sub-C cells here, plenty of places sell them and the good ones will produce crazy current all day long. The Component Shop has a good selection as do many others. They are available at the same capacity as the Cs I use and are a bit smaller which might give you more flexibility in design. Three of them direct driving an SST-50, maybe with some resistance put in to keep the current sane ought to do the trick. You'll easily get three SubCs in a 2D Mag with plenty of room for a huge heatsink. If I had access to a lathe I'd turn a big chunk of copper so it is a tight fit in the Mag body then thermal glue it in place.
The stock Mag switch should handle 5A with no trouble - they used to be capable of 11-12A but I suspect recent ones are not as good.