The light head is a standard 3D maglite I found on ebay, it had a rotten battery in it that was stuck but this didn't matter for my use.
The body of the torch is quite soft metal so a junior hacksaw was enough to cut it, cut just after the switch assembly so as to preserve the end of the torch.
To Disassemble the torch have a look on the web for instruction or at the link below.
The Head of the lamp is screwed to the body of the torch, the cut end now fits into the torch grooves.
I found it didn't go in a long way, so ground the threads down with a small grinding wheel. It didn't make it go in a great deal more but enough to jam it solidly and glue with the liquid metal (JB Weld). This was liberally spread onto the two mating surfaces, the cut end of the torch body and the inside threads of the torch head.
The picture below shows the resulting join between the head and the shortened barrel.
Here is a view from inside the torch head, the liquid metal sort seeped in quite a bit to cover the join and cover the ground down thread of the inside of the torch head.
The end cap was drilled out to accept the cable gland. The material used on this end cap was not so soft and I had to ask a friend to drill it out as my meagre drill slipped all over it... Recommend you have a proper bench drill for this.
The end cap screws off, I had purchased a new o-ring to replace the one that was there originally,however, in the end I decided to use instant gasket material, apply wet and screw up, no o ring.
The logic being, I can unscrew it and start again easily < update near impossible to get off !>
Apply silicon grease to the cable and o-ring within the cable gland.
The o-rings in the head unit are all upgraded ( see parts list), there are three, one around the head as shown below....
A second o-ring in the lid or rim of the head.....
The third sits on the lense itself in the groove , this is the underside of the lense or the edge that faces in.
The original lense from the maglite was a flimsy piece of plastic so I replaced it with two pieces of acrylic glued together, one with a slightly smaller diameter so that the o-ring is kept aligned to the torch head.
Here is a side view with the o-ring mating to the torch head and the lense.
When its all tightened up, the first o-ring should just be visible.
All o-rings are greased up with silicon grease.
The light head I choose was driven by the desire to have as many lumens are possible, in the end it probably wasn't the right choice as it took an awful lot of grinding to make it fit and i was lucky it still worked afterwards!
I've no metal working tools are such, only an angle grinder I used for cutting some concrete posts! But it can be done, put the light in a vice and gentle grind away. Stopping frequently so as not to let it get too hot.
Keep comparing the size of the drop-in with the opening in the torch, it will need a lot of grinding to get rid of all the fins...
Here is what it looked like before I ground it up!