Check the Container Store. They have all kinds of special storage products. Their Elfa system has some pull out drawer sets for small undercabinet areas like bath vanities. I have one and find it holds a lot of useful bits. Online shopping is www.containerstore.com I am not connected to the company in any way, just like their products.

Thanks, i’ll Take a look!

There are some other blind-corner cabinet solutions.

Here's a pic of one, from Houzz even:

https://www.houzz.com/photos/4314973/Blind-Corner-Cabinet-Solution-cabinet-and-drawer-organizers-indianapolis

a video

https://www.youtube.com/watch?v=_IoBHKV7o-A

Here's someone's Pinterest board; there might be others there

https://www.pinterest.com/WalkingWomanCO/blind-corner-cabinet/

Shelf Genie makes one; so does Häfele.

as does Knape & Vogt: https://knapeandvogt.com/slide-out-base-blind-corner-unit?

Most of them (eg., K&V) need a 14" opening, so as you point out, that doesn't help.

But I wonder if you can separate hardware from shelf, and use wooden shelves with their slide hardware...

Hi! I know it’s just been a month, but any chance you’ve found a solution? We have a blind corner cabinet with the same size opening! So frustrating to have this much wasted storage space!

I haven’t found one yet. but I also haven’t explored any of the DIY solutions anyone has mentioned yet, because I am leaving that area of my kitchen re-arranging for last Since it’s the most frustrating.