My plan is to wire up 2 (maybe 4) 18650 batteries to power my P7 to extend the run time. My new commute to work will be about 45 minutes, and while the Medium setting is more than enough, High is killer sweet.

I need something that I can strap to the frame quickly, and remove the batteries easy once off the frame. Since most of the 18650 chargers appear to be a 2-battery setup, I'd probably stick with two.

The plan is to drill a hole in the the P7 to allow the leads in with the negative lead being a little longer so that I can unscrew the bottom if needed for any reason (as well as for access when soldering).

Where can I find a small, weather-proof enclosure, with an spot to insert a velcro strap, that will tightly hold two 18650s (stacked or side by side)?

Why not just buy another $8.50 charger off DX and leave it at work? You could even get 2 more batteries if you feel you might need even extra run time. They would easily be recharged in time for your commute home. Then you could run high both ways and not have to deal with extra batteries and wiring a harness together.

I haven't yet figured out how long my MTE 8-modes last. I did a 75minute ride the other night and had one light on high for +90% of the ride and it still seemed like it was going strong when I got home. The other light was moved from off-medium-high throughout the ride.

ask wiggle, he's got the sort of setup you're looking for.

https://www.bikeforums.net/electronics-lighting-gadgets/480616-dual-mte-p7s-modded-use-battery-pack-2.html#post7736124