I might buy another piece of wood and section a bit more but it works just fine as it is:
The only issue that I can see is that small stuff might get under the slats. If that happens I'll get a think piece of wood and add a bottom making it an entire drawer inside a drawer insert. But I don't think I'll need to do it.
Funny thing is that the drawer is not perfectly rectangle. The insert will only work one way. Fortunately I measured and tested enough that it is a nice custom fit.
All in all pretty easy. And nicer than buying plastic pieces. I might even get wild one day and stain it so it looks more like the original.