First, spare parts are kept separate from complete guns. I have found when a part is needed, you do not remember where the spare part bag is. My solution is to have a spare carbine. Of course, then that one willl require need a spare also, and so on. It never ends until your money or marrage does. Then the ex-wife gets half the spare carbines. You get to start all over again. Did I say, it never ends.
Second, I have found the best way to keep spare parts is assembled into a complete carbine. You can lose a small bag of parts, but not near as easy lose a complete carbine.
Third, If you are dead set on getting spare parts, I would suggest assembling them into sub-assenblies. Small bolt parts into a complete bolt(headspaced checked to your carbine). Trigger housing pins and springs into a complete trigger housing with a housing pin glued in.
Of course, with the expense of all the sub-assemblies, a little more and you can have a complete spare carbine. Have I said that already?