Here is my doujinshi collection for UsoKa. While there exist quite a few Usopp doujinshi out there, surprisingly few of them feature Usopp x Kaya. Outside the two listed here, I'm only aware of one other that I've yet to find.

If anyone has information on any doujinshi not listed here, like a picture of the cover, summary, or even just a title, please email me.

Voices
Series:	One Piece
Circle:	Rengaworks
Main Characters:	Usopp, Kaya, Usopp Pirates
Rating:	A
Size:	B5
Pages:	46
Published	April 29th, 2003
Acquired:	June 20th, 2017
Summary:
A pirate arrives in Syrup Village and the Usopp Pirates confront him. Thinking about Usopp's bravery inspires Kaya to step up and protect one of the boys and gets hurt in the process. The villagers step in to get rid of him.
Review:
Not much of Usopp in this one, and it's mostly just him fighting during the Alabasta arc. Love seeing a tougher side to Kaya though. This story inspired me to write a similar story about pirates attacking the village and Kaya having to step up.
Lady Blossom*Shine
Series:	One Piece
Circle:	Sae / Sight
Main Characters:	Usopp x Kaya; Koza x Vivi
Rating:	A
Size:	B5
Pages:	60
Published	October 12th, 2003
Acquired:	December 11th, 2017
Summary:
While Nami teases Usopp about Kaya, back in Syrup Village Kaya is worrying about him. Merry comes running in with a bounty poster for Usopp and she hugs it, happy to have news about her friend. Only after looking at the picture closer does she notice written on Usopp's armband is her name, which only makes her happier.
Review:
The UsoKa part is actually just a 15 page side story but very cute. The majority of the doujinshi is KoVi, and while I do support that ship, it wasn't why I bought it. I got it for the UsoKa and that's why the summary is focused on it.