I agree, for the average person its way easier (and possibly cheaper) to just buy one. The downside of a 3pt splitter is the expensive 540rpm pump. If you can get around that, i fail to see how it wouldn't be much cheaper to build.
That being said, at some point i hope to build my own. I don't split a lot of wood and what i do is mostly pine so cheap is key as i just split to make it burnable in my fire pit.
No plans but here is a pic of what i would like to build. I would run it off of my rear hydraulics and live with the slower speed of not having a 2 stage pump. Primarily for the lack of maintenance of not having an additional motor or expensive PTO pump.