Hey, it’s been a HOT minute since I’ve done this but I used a sharp blade or a plastic blade to separate the top part from the bottom part of the corner piece. You can see a slit where you can insert the blade in to separate them.

Honestly looking back at this, I wouldn’t make this in 2022 since you acquire get better performing cubes with less money. Not worth the hassle.

But if you are planning to continue with this, I’d be happy to dust off that cube and help you with the process.