This is not a new design, but I haven't seen it broken down into a how to before, so here goes:

Gather your ingredients (These are approx RRP):
Test tube 150mm Schott                                          $1.00
12v 50w Transverse Halogen capsule bulb              $13.00
Cable @ 0.98/m                                                        $9.80
Cigarette lighter plug                                               $2.00
IP68 electrical Polyamide cable gland 3-6.5mm x 1  $3.00
IP68 electrical Polyamide cable gland 13-18mm x 1 $6.40
Superglue / Araldite
Touch of silicone sealant
Soldering iron
Beer

I had finished the beer by this stage

The domed locking collar of the big gland will probably have to be opened up slightly to give a tight fit over the test tube - this isn't a watertight fit, so don't worry about going slightly overboard.

 With the rubber washer and locking ring in place on the small gland, apply adhesive of your choice and jam it into the locking ring end of the larger gland. Once cured, carefully apply a small bead of silicone in the recess (facing you in the following pic) up to the base of the thread of the small domed locking collar. I have used 5 minute araldite for this quite successfully, by introducing the glue, then warming the assembly so the araldite thins and flows ready into the bottom of the collar.


Next solder your cable to the halogen bulb - I use 12v 50W units. Try to avoid handling the bulb capsule with fingers as the oils and acids at the high temps apparently have a detrimental effect...so they say.  Keep the ends short and you won't have to worry about sleeving the contacts.  Use a round cable if you can, I used a 2 core light mains lead here.

Crappy soldering innit

Feed the lead through the gland assembly with a sniff of silicone where it sits in the small cable gland, then slide the test tube over the bulb and with a touch more silicon clamp it GENTLY into the gland.  Try and have the bulb sitting so it doesn't touch the sides - I have previously used a small wad of pink batts around the cable/bulb connections (not shown in this one).  Whatever you use it must be rated for mega temperatures.

Run the cable for as far as you want and plug it in. Instant nuclear fishin !(very weak pun)

Points to note:
Turning it on then lowering into water will result in a pop and it all goes dark.
Pulling it out of the water before turning it off is likely to have a similar result.
QI Bulbs are VERY sensitive to being dropped, nudged or even looked at the wrong way - treat with kid gloves when out of the water.

I have used these to around 2m without any problems, but don't see why they shouldn't go deeper if you're after giant squid.

Lethal hanging out of the scuppers when floundering in the yak.

PM me if you have any questions or want something clarified.

Cheers
HLD

HLD, Just wondering how much you spent on your canoe?