One thing that no one has brought up is weight. That little Keltec .380 weighs a LOT less than a G36 .45 weighs. That little clip might work well on the .380, but I doubt you will like it on the much heavier .45. I would get a good holster for sure, wether it be a IWB or belt slide, you will better support and weight displacement than you will get from a little clip.
I have been carring concealed for 16 years now. My closet shelf is full of what I thought were good ideas, only to find out in real life they didn't work out so well. I pretty much now only carry in a Cross breed or a Tommy Theis IWB holster. I have them for my .380's, my Glocks, my Sigs and my 1911's. I would also recomend a good sturdy gun belt.
All of this is just from my experience, so take it or leave it.
Best of luck in whatever you choose to do.
SGT U.S. Army Special Operations Command, Ft. Bragg, NC (2002-2006)
Operation Iraqi Freedom Veteran (2004)
"Democracy is two wolves and one sheep voting on what to have for dinner. Liberty is the well-armed lamb contesting the vote." - Benjamin Franklin.