Hemi,
You don't state if your custom lights come equipped with low voltage protection. All of my lights (I do not mod or build yet) do have it and is a requirement for me unless I were to plan on modding the unit. Low voltage in a light is not a necessity for everyone but a required preference for me.
With Christmas coming in the somewhat near future (ugh) I plan on gifting quite a few lights and had to ponder batteries. I am torn and think that it would be cheesy of me to give people lights without batteries (although I would rather because of costs). For others, as an added layer of safety but not required I prefer to give protected batteries. Personally, from I have read, I do not even trust protected batteries. I instead try and utilize a quality charger and a a quality light. That is what I do for my lights I use.
For a friend of mine I purchased the following just for the hell of it and curiosity.
http://www.amazon.com/gp/product/B00LC0ARZA/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
I never heard of these before but $20 is the cost of 2 decent batteries so why not try and see.
The result was that through VERY crude testing (I do not have any hobby chargers etc....just a quality DVOM) that these batteries held up by providing very ample run time. How reliable is the built-in protection - no idea because my low voltage protection kicks in before the battery drains to unacceptable levels and the charger stops at 4.2V.
Here comes the gas on the fire. After some research it would seem that these batteries are indeed manufactured by Ultrafire. Is Dulex a separate company subbing to UltraFire with mandated specs or is Ultrafire trying to put out a decent line of batteries - I do not know. I just know that these batteries seem to work and I have no regrets nor guilty conscience. For myself, I still prefer my unprotected Panasonic 3400's.
Two more points and no more soap box.
1. When people (other individuals NOT you) look at the cost of these rechargeable Li-Ion and think they are expensive they show their ignorance and the fact that they have not purchased batteries in a long time. Tell them to go buy a 8 or 10 pack of AA or AAA alkaline batteries and see how long they last AND how much they cost. 1 Package of batts equals 1 Li-ion that can be charged over and over.
2. Lastly, if protection is not a concern and you just need batteries for people to play with (used loosely) take a look at these threads as they are well worth the money and the gamble in my opinion (and yes I have bought and plan to purchase more)
Both of these guys are jam up good guys and there is no way in hell to find batteries cheaper than this unless they are stolen! LOL
Good luck with whatever direction you go!