Pretty sure this is the closer fit you can get if you want to use these keys. If you mean an higher wall, well that would be great, if you just mean closer, believe me this is already too tight
I won't
if you make it tighter, the cap pressed wobbles, by wobbling it will hit the wall (also not all caps are exactly the same dimension, most of them are around 18.30mm but some might be bigger, there are new coming).
The space between caps is about 0.75mm meaning that gap you see is 0.30/0.35. If you have it, take a calipher and check how much is small 0.35mm please, that's the distance from the cap and the wall if the wall is high like the cap. The only thing OP could do to reduce the gap from the angle, is to make the wall higher (but probably is already high enough and good like it is). Imho he did a good job with that wall distance, if smaller, it will be risky and most likely faulty for someone.
@gok101 feel free to correct me if i'm wrong with the quoted measurement
