I think the first question is the rate at which you can pick up spell fragments.
As this determines how quickly you could open the box.
I think me and enough other people would not care nor wait for a good thing to make.
If I got enough spellfragments I would just make essentially anything to fil up that box. There aint much point in collecting millions of spellfragements and not using them.
If I would have the catalysts I would make this:
If I dont I would make this.
or this
Of course you would save enough fragments in case somthing good comes along. but all excess are there to throw it in there.
There would be little point not to do it.