Using a chopping board from IKEA as the basis, along with a few common fasteners, this is what I came up with:

This jig does precisely what I'd hoped it would do, and only cost me about $5 Canadian. Being made of high density polyethylene, it was easy to machine and resists all chemicals a scale modeller is likely to expose it to. Even CA adhesive won't bond to it (thus, the need for assembling it with screws).
For those interested in a more in-depth account of how I built this jig, I've posted a video on YouTube: https://youtu.be/BGDpVvuYbUs
Cheers!




















