On this one you should start by cleaning it up, then take sandpaper or emery cloth and snake it through the gap and try to ever so slightly take it down and even it out. Then hit the clean bore with fiber grip and see if it just works.
There should be a hole in the bottom of the slot so the ears can flex correctly. If not or if it's too minor, that can also be a simple fix, but be careful.
If it has a proper hole at the bottom of the slot, don't try to extend it more. Going further with the slot so close to the joint is a bad idea. At that point the solution is to turn it an h-shape binder slot by adding another slot perpendicular to the first at the bottom, and then another hole to avoid stress risers at the end of both ends.
Edit: Also before you do anything metalworky on the bike, you should get to an absolute understanding of what's really going on here to cause the problem. A seat tube opening made out of spec can in fact cause this, or a slot that's too short, or a slot that doesn't have its little relief hole at the bottom, but issues with the seatpost being too small either in nominal size or being under spec to its nominal size are all much more common.
Here's to give an idea of what I might about the perpendicular slot, in case it really is all an issue with the slot it has not being long enough.