Yes, crates. This might seem a bit too basic to request a concept for, but I believe crates, barrels and doors are the most important map props as they are the ones you interact with on a regular basis.
While technically map objects don't use a bounding volume for hit or collision detection, it's desireable that their surface can be clipped flat (→ no details sticking out of it but there may be concave areas) so that wallwalking on them feels smooth. Both crates and barrels should also have a flat top, so you can use them to climb or build structures on them. Two useful crate proportions are (lwh) 443 and 446¹, since these can form nice steps for humans and at the same time provide enough surface for building.
If you want to you can sketch the props in an environment but this isn't necessary.
¹ In Quake Units this would be 64x64x47 and 64x64x94. The -1 and -2 ensure that humans with a jump height of 48 can always climb them easily, even when there are framerate or rounding issues on either client or server side. (Every frame the position of a player is calculated as a floating point value and for certain frame rates the area where a player's position is > 48 isn't hit by any of these samples.)