It's shaped like a box and is smooth. The percussionist sits on top of it and taps at it with his hands and fingers. I'm also pretty sure it's hollow.

Would you actually just call it a box or what?

Thanks in advance
yeah, it's gotta be a cajon.
There's no such thing; there never was. Where I am going you cannot follow me now.
I 3rd the cajon. It's a south American instrument. People would take things like dresser drawers and shipping crates and hit them with their hands because they couldn't afford drums. Later the instrument evolved and got simple snares which gave them a sharper tone and the shape evolved to make them easier to play. They are very simple instruments that actually NEED the batter head screwed on rather than glued and they need to be made of plywood and not expensive exotic "tonewood." I know because I've made several myself and the worst ones in terms of construction and materials turned out the best in terms of tone and longevity.
Not taking any online orders.