Not sure which iteration of the M951 you have, but here's how I got one to work, without any boring whatsoever (this M951 had the newer M2 head with hex anti-roll, and the LU60 adapter with the 6 semi-circular cutouts to aid grip when removing).
You'll need to remove the black plastic spacer ring thingy inside the LU60 adapter collar, that has 6 tabs holding it in place. If you don't want to break the tabs, you'll need 3 toothpicks - break each one into half and use the halves to hold the tab open after lifting it with your fingernail. When all the tabs are lifted by the toothpicks, just push lightly on all 6 toothpicks halves at a go, the plastic ring should pop out.
Next step is to make the spring on the Malkoff longer. You can either solder on an extra bit of spring, or carefully pull on the existing spring on the Malkoff. I (carefully) used a magnet on the top of the cell to check whether it will work before lengthening the spring. A magnet isn't recommended because of the possibility of moving and contacting the walls which are only millimeters away with the result of shorting out the cells.
BTW, there is no such thing as a P90L, SureFire never came out with those.
Also, a warning on using the full-powered M61 in the unbored M951: there is minimal surface contact between the brass body of the Malkoff and the aluminium of the LU60 collar - it's good enough for electrical conductivity but I'm not sure if it's optimal for heat transfer.