Scrap saw, preferably an old one that doesn’t have a laminated blade, a sharpie, a square and a hacksaw….voilà instant cabinet scraper. You will have to true up the edges with a file but then again you would have to file store bought one as part of the sharpening process.
A little tip, as long as you aren’t scraping tiny intricate surfaces make your scraper large, as large as you can get away with. It’ll stop your fingers getting burnt when you get a bit enthusiastic :-)