How do I make my hitboxes work?

I believe directly adjacent bounding boxes are considered colliding/overlapping. Even if they weren't, rounding errors would ensure that exact fit shapes like this would never actually fit together.

I've seen some solutions use tile movement, others disable solids/collisions (or adjust the bounding box to be smaller as you mentioned) and implement a snapping system. The platform behavior by itself will likely be insufficient to handle perfect fits like this.

What would you recommend other then platformer? Tile wouldn't work in this situation.

I imagine you would use a snapping system, when you got close enough, to manually position the object in and disable platforming. Or give yourself some wiggle room so the fit isn't so tight.

I mean even in real life, if you had two shapes like that that were EXACTLY fitted, it would be hard to connect them. There's usually some wiggle room or flexibility, even if very little.

It depends on what the goal is. Is it just an obstacle you'll pass by, or is it an object you want to "dock" or connect with?