I recently finally finished this, based off of plans I downloaded over a decade ago:
The original design had a compression spring under the tracking/tension arm, but based off a commercial model I bought, I redesigned to to use the extension spring as shown.
Works great, but before I managed to finish it, I found a brand-new Wilmont in a local secondhand shop. So I finished the homebrew one, and sold it to a buddy. He's already used it more than I've used the commercial one- and I've used it quite a bit.
As for tracking issues- parallelism on these things is
everything. That's partly why a lot of people recommend "no weld" builds. If any one of the rollers gets canted out of line of any of the others, tracking gets a little wonky.
In welded builds, unless you're extremely careful, everything gets bent, bowed and wowed- not a lot, but it doesn't take much. Not easy to fix, either.
Doc.