I recently took a course on plane sharpening with a local Woodworkers Guild I belong to.
The instructor sourced stone/granite counter top pieces and adhesive backed sandpaper through 600 grit (2 3/4" wide rolls, very convenient). After flattening the toe/heel/mouth of the plane sole, then cleaning up the frog to sole contact areas, we worked through ~1500 grit paper (above 600 on a flattened granite plate).
I too had that $15 (on sale price of $10) little jig you opened with. I confirmed that it indeed is not square along the straight side of the clamp (with the roller). I ended up with a skew edged plane iron -- one person in our seminar had he original Lee and another the version you demonstrated. Both look very useful -- and are money well spent over the $10 rolling junker.
Sometimes you have to spend enough money.
![Crying [crying]](https://www.festoolownersgroup.com/Smileys/default/crying.gif)