A very niche idea...

I'm cheap.
Gutters are cheap.
Gutter end caps are not , in my opinion cheap.

I mostly avoid the issue by putting the gutter on pronounced slope,but I had an idea and acted on it:

In our previous house someone had cut a suitably sized log up into disks and then used them with some silicon. it worked and had been working for some years. Our gutters had a round profile.