Understairs space to fill- Any ideas?

You can add pull out shelves to the interior. The example attached is a kitchen cabinet but can work for your project also. You can build or purchase ready made slide outs for full width or half with two per level. You can build diy and add full extension glides so you can access everything easily. Good luck with your project

Lots of ideas here:

So you could make something like this, but put a triangle facade on the front instead of the rectangle in this picture.

PS and you could just put it on wheels so you don't need to make a cabinet for it

I'm not sure which side of the stairs you want to build your drawer, so I'll send you several videos to choose from. I always thought drawers in stairs was such a clever thing! Let me know what you do with it!

These are a couple of ideas. I didn’t want to diminish your beautiful cabinet, and many things that might fit I wouldn’t use in my decor. Or yours.

I particularly like the set of three. if nothing else, these might inspire a new mindset.

Here is an article that might help - https://diyprojects.ideas2live4.com/under-stairs-pull-out-storage-how-to/