There are commercially available keycaps for the Kailh PG1316S which are perfectly compatible with the Cherry MX ULP. I have both switches and tested the keycaps on both. They fit perfectly.
I got them from holykeebs here. I'm not affiliated to them in any way, except that I bought from them. I wasn't able to find other vendors at this time.
This might be worth mentioning in the keycaps section.