Got the PCB layout done and added to the project files. PCB is two sections that are cut apart before assembly. Left side is the probe front end with a large blank PCB tap to glue/tape to the switched 4xAAA battery box powering it. Right side is the probe back end and power supply. The mounting holes and header A4 allow multiple boards to be stacked using only one power supply. With the 12Vac 500mA power cube specified on the schematic, you can stack up to four boards and only one board needs the power supply parts populated.
On the probe head PCB, A2 and A3 are large solder pads for signal input. The pads are designed to have jumper wires, spring-pins, rigid probes, springy probes, etc. soldered on.
Back on the back end amplifier, trimmer R23 adjusts for the DC offset while trimmer R26 gives control over the gain. At the moment I expect both offset and gain will need to be adjusted soon after the probe is turned on.