Today I bought a used 18mm auger bit, (Cost $8.00) which made nicely press-fit holes to take the 19mm jaw piece projections, now fitted as shown at left. It was not easy drilling into end-grain though!
I looked at adding the guide plates, but although I have heaps of screws I could use, I want ones I don't have enough of.
Getting techie with screws
Round heads are no good unless the shaft is exactly the hole diameter, to prevent the plate moving sideways, and then the domes would be too big.
Oval screws, on the other hand can have heads just bigger than the plate hole, will only stand proud without an edge, and can sit at a bit of an angle if not spot-on aligned, yet still pull in to the edges of the hole and keep the plate from moving sideways. They are the ones for this job, I reckon, even for the back where they can't be seen.