How to make 3D colliders on sprites?

I'm making a 2D beat-em-up and doing some camera trickery to make everything look OK. I'm trying to establish a "floor" for my character to walk on, and I'm attempting to make a collider to surround the floor with. Of course, like everything else I've been doing in Unity as of late, I just can't catch a damn break. I'm extremely frustrated at my inability to solve something so simple, but I just don't know what else to do.

I bought the 2DColliderGen utility, and it is able to make a mesh collider that fully encapsulates my sprite. However - and this is the reason for my rage - IT DOESN'T COLLIDE WITH MY CHARACTER'S BOX COLLIDER UNLESS I MAKE IT CONVEX. This is important, because making the collider convex completely changes its shape and it is no longer representative of the sprite I am using. I've read dozens of posts about how mesh colliders only work against other mesh colliders if they are convex, but this is a case of a box collider failing to detect a mesh collider. Upon further investigation, I have discovered that my mesh collider isn't actually fully 3D - it only encompasses the perimeter of my sprite. I want something that completely wraps around the sprite - the ideal case is a collider that is a slightly extruded version of the sprite itself. As far as I can tell, there is absolutely nothing in the 2DColliderGen that is capable of doing this.

I don't want to go and blow more money in the Asset Store yet because I'm thinking there's a way to do this. I've tried the free UCLA Mesh generator, but this also doesn't work - it will create a usable mesh that collides, but it always changes the scaling such that it isn't 1:1 with my sprite. I swear to God, it's always something.

Somebody, anybody, if you have experience with this I am begging for your help. I'm seriously on the verge of giving up because it feels like the stupidest things are constantly holding me back.