After looking at the companion cube designs here, I decided I wanted to make my own design. I wanted something that I could print in pieces and snap together without too much difficulty. Parts were created in inch units so when you import them to a slicer, you may have to account for that. For me in Cura I had to set it to 2540% when it made everything in mm. Just make sure you set that so the cube is 2 x 2 x 2 in.

Print:

1x CCube_Cube_Center_Full.stl
8x CCube_Large Corner 0.0156 shifted off center.stl
12x CCube_Edge 0_0625 (thinnest).stl
6x CCube_Heart 0_22.stl
5x CCube_Circle 0_495.stl
1x CCube_Circle 0_475.stl

Once parts are printed, pop the corner pieces into the corner holes, the edge pieces into the edge holes, the circle centers into the center holes (the 0_475 circle goes into the side that printed on the bed), and then the hearts into the hole on the center pieces. Voila!

NOTE - Alternatives for Corner and Edge:
Choose the smaller corner if you like that more. Also choose which edge thickness you like. For reference - In the picture of the two cubes stacked, the darker cube is using the 0.0781 in. thickness edge, and the lighter cube is using the 0.0625 in. thickness.The original cube was using 0.125 in. thickness. I liked the 0.0625 and my partner wanted one with 0.0781.

NOTE - Circle difference
The reason there are two different parts for the circle part that goes behind the heart is because of the tolerancing issue I have with printing direction. The center holes on each side are 0.5 in. so most of them fit with the 0.495 in. diameter circle, including the top side. The bottom hole gets squished a bit so I made a copy of the circle part at 0.475 in. diameter which consistently works for the cubes I printed.

I'm definitely curious to know how it prints with other filaments and settings, if it's as consistent as I got on my printer. I did a lot of prototyping and caliper measurements to get the right sizing for what I have.

Other Notes:

The edge pieces between the corners are bigger than they would be scalewise on the original companion cube, so I might change that later and update, but I wasn't aiming for a precise model on this one.I just used my judgment in how it should look.

I didn't have any pink filament but I had this old roll of purple that turns pink when hot, so the hearts on my printed model are purple-ish pink. I also had some old granite color, which is what inspired me to model the companion cube, besides Portal being one of my favorite games.

I'm going to make an updated print when I get dark gray and pink. I will probably try playing with the sizing of the corner and edge pieces then, but right now they all snap in well and I think if the edge gets much smaller it might be prone to breaking.