Slimagainsoon's mounting idea here had a lot of merit but after looking around I think you have to luck into finding something like that fishing mount. But some of the concepts were great and stuck in my mind.
Lots has been written about there, but for those who werent paying attention, here are some beamshots from 7yds away in the dark. Top is the Ultrafire sitting on the seat, middle is a 9 LED blinkie, bottom is a SF on steady. One shot is directly behind, the other is off to the side a bit to reduce flaring. The 9LED is more "orange" than I thought, but it always runs in a blink mode.
My approach was to fashion a plastic 'plate' for the inside base of the bag and cinch some zip ties thru it to a modified TwoFish on the outside to hold the light.
I used a legit TwoFish FLashlight mount as opposed to the DX conterfeit. The fake one uses 2 slots and is a bit smaller. Since a strap as well as 2 zips need to go thru the strap slot, the larger mount allowed more working room. The modification is simply to cut most of the arms off one side so it sits closer to the bag. A dremel seems to work better to make single, even cuts vs even a sharp knife. What remains is a very slight arc on the underside.
For the base plate, I used a piece cut from an 4xD Cell holder. I put 2 holes directly under where the TwoFish would sit thru the bag and base plate and zipped them tight. This was mainly to hold the plate to the bag for drilling the main holes, make sure everthing lines up the first time and eliminate extra holes. The zip could be removed later but the lock sits under the small arc remaining in the TwoFish so I left it.
Two pair of main holes are drilled close to the mount and at the front and back edges of the strap slot. This allows the zips to use the edge of the slot leaving room for the strap.
Some may be able to use the original long LockBlock strap thru the slot, around the light and then around the bag back to the mount somewhat like they are intended. If the zips should fail, the strap should hold onto the light and mount at least for a while. This was not my intention and my bag is too large that close to the mouth, anyway. I just used a short strap from a DX/KD fake to hold the light on the mount and a longer strap all the way around the bag and light combo closer to the tail. This might help a little if it comes out of the TwoFish, but mainly it does pull up on the tail of the light to focus it.
The UltraFire comes with a real sissy lanyard, so I threaded the seatpost straps of the bag thru the lanyard loop. It is not much but it might hold long enough for me to notice an escape attempt should the zips break.