Get a finish carpenter to cut wood to cover those “holes”. It should look like an original installation, not like a loose piece of wood lying there.

Agree, carpenter will be able to make a piece of wood - then paint it white.

cover entire flat surface,with mdf then paint white to match. what else are you willing to do?

Some options include:

My preference: Have a thick wood board as table top built for it -- in a wood finish you truly like. You then have a side table for any seat next to it, you have hidden storage and you don't alter the "style" of the house for a future buyer.

Cut out the one side of it to create an open bookshelf there.

Find a plastic planter that has a bottom that fits the space and actually use it as a planter.

https://plantersetcetera.com/product/salon-narrow-low-rectangular-planter/

Cut out one side and create a single thickness pony wall with a niche space behind it.

Cut it out entirely and leave the space open or set a floor lamp or narrow table and lamp there.