I made a 4-frame extractor with a friend of mine. I’m not posting the plans for it because it’s a prototype and the design has some flaws that need to be corrected first. But it works and is easily worth the $120 I spent on it. Here’s a demo video of its maiden voyage:

By the way, the heating gun method of uncapping the honey works great. No fuss, no muss and way cheaper than an uncapping knife.