Sigma's 120-300mm has been made in several versions. The true original version (non-OS, non-DG) had the TS-21 tripod collar, which the Kirk LP-2 should fit. The DG and OS versions had (I think) the TS-41 tripod collar, which needs a longer plate - I can't say which one for sure as I haven't tried it.
I use an RRS L97L plate with my 120-300 DG. I don't know whether that's what RRS recommends; it's slightly shorter than the foot, although that hasn't proven to be an issue for me. (You can reverse it so it runs from the front of the foot back if you prefer.)
After another go around with Kirk it's their LP-54 plate. It fits a lot of the Big Sigma's and extends over the foot by about an inch to I'm told accommodate a flash bracket(why would any one use a flash on this I do not know).