I believe RJ the Bike Guy also has a video on the slide hammer method. It's all hardware store stuff, maybe 3 feet of threaded rod ("all-thread"), nuts/socket/washer on one end, weight/washer/nuts on the other. A socket can be used as the sliding weight as well, if you find a heavy enough one.
BTW, using any twisting method, be prudent. I don't know the exact wall thickness of that tubing, but as you know, it's no gas-pipe Schwinn. There's a thread somewhere here in C&V, or maybe in Mechanics, depicting a nice late '80s Trek frame with a seat tube kinked/twisted by a high-leverage twist effort to remove a stuck seat post.
One method I've wanted to try (I happen to have an SR Laprade seat post stuck in an '86-ish Tempo frame) was some sort of moderate twisting force, but maintained over time. Some way of holding the frame in one position, then turning a lever attached to the post and holding it in position. My hypothesis is, I've seen sustained force like that work on things like press-fit steering ball joints in motor vehicles. I have a screw-type ball joint separator that often works best simply by cranking down on the screw, and just walking away from it, and several minutes later you hear it POP. I imagine the force is somehow working over time to separate the bond between the two parts, and I wonder if the same could be applied in a case like this. I just haven't figured out a way to set it up.