Go for any of the Ultrafire XM-L drop-ins. The The others will produces to much heat.
The one i would chose is the Ultrafire 5 mode. The 3 mode( without flashing) is good but some have received under-driven drop-ins, They are not driven to 2.8Amps.
None off the drop-ins can take 2 18650 batteries as then are limited to max 4.2V. If you want to use 2 batteries with an extension tube, you may be better of getting the KD 1 mode Drop-in's.
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