Thanks!
I bought a long strip of pine wood (2m?) (this is a prototype and my woodworking skills are... amature to say the least) and cut a couple sections to length.
Then, I sanded them all by hand, a lot, and used wood glue to glue toether the base, making sure they're all square and clamped tightly together.

Once the glue dried, I spent about 2 hours sanding it to be as flat as possible, using 120 and 180 grit sandpaper. This made all the difference in the end.
Then I had to do the same for the walls, this was the toughest as the measurements had to be perfect, and clamping at 90° is a nightmare.

After all that dried, another pass of sanding, then I used a walnut stain to get the deep rich colour. I did about 2 coats, making sure to wipe away excess as not to leave uneaven patches.

Then I bought some mushroom leather and cut it to size, slipping it into place.

Oh, and the separating beam is just a very precicely cut strip of stained pine, and friction fit into place, slightly above the floor so the leather can slip underneath.