How to tie a cube securely?

Good morning! I have to tie a 1"x1" cube so it can be suspended from a fixed point. I cannot for the life of me figure out how to do this securely. I have had some limited success with a barrel knot, but I'm unable to cinch down on the top of the cube, and the thread end up slipping around the cube sides and the cube drops.

Are there better options? Is there a lashing designed for a cube? Is the barrel knot a good choice and I just need a better knot to tighten everything down?

I'm using a thin cotton blend thread, the cube is less than 60 grams, object weight and rope strength are not issues, I just don't want it to slip out.

Thanks!