Building lithium battery pack advice

Well ok not just advice but lots of it if possible guys.
Firstly is there any of the modders on here that actully build packs for people?
If not I don't know what I'm doing but i'm more than willing to give it a try.
I'm trying find out what I would need to put together a lithium ion pack to power a 12v 35w bulb for as long as possible,its basically to power my lamp for night shooting all on footand i'd like it all to fit in a box with internal dimensions of L130 W68 H46, any suggestions as to what sort of power rating I could squeeze into this with all the electronic trickery. I don't know what all the pcb/pcm/cmb do or even if they are all needed but hope someone can help me out there. I know using 18650 maybe the way to go and they arn't that expensive so its down to know how and then the electronic parts and so on.
I've looked at some of the battery space ones and either they are cost prohibative or mostly size issue and also the discharge I'd like a connector that deben use as I can then just plug and play with he lamp. Any and ALL advice greatly appreciated as always.
Lastly if the box size is a real issue to fit things in then I guess I could go very slightly bigger if it made things easier.
Many many thanks all

If you can use tabbed cells, it'll help you greatly. If you can't you can solder directly to them, but it's not something that should be attempted by people with no experience with soldering and/or LiIon cells, as it can be dangerous.
I wrote a guide about soldering to cells some time ago, you can find it here; do take notice about the safety warning and disclaimer. I've done it many times and haven't had anything catch fire or overheat, but it still shouldn't be done if you're not prepared to deal with a possible lithium fire - you never know, everything could go perfectly well for 100 times and then the 101st cell catches fire.

If using non-tabbed LiIon cells, and if your light doesn't eat much power, it's safer to use holders - there aren't any explicitly meant for 18650s as far as I know, but you can hack some up out of C-cell holders, plastic and some glue.

Also, if you plan on putting several cells in series, you'll need to balance them. Ideally this is done with a proper balancing charger that always balances, but in a pinch it can be done by disconnecting the battery from whatever it's plugged into every few charges, and charging every cell separately. It's a good idea to build balancing taps in your battery; connect a wire to every inter-series connection, so you can monitor and charge each cell after the pack is put together.