Based on some inspiration found on "ze internet" I realised I could make a pretty simple jig with my lasercutter. It's a chinese diode laser, so it cannot cut really thick materials, but 6mm ply (which I often use for templates) is fine.
So I made this:
It is a lasercut baseplate, doubled up to give it a bit more sturdiness. with a bridge fitted with magnets to guide the saw.
I use something similar to saw dovetails (I can do without, but they turn out tighter if I use the jig, its a DIY Katz Moses jig). It is simple enough to make if you have a lasercutter, and accurate enough. Even for guitars, and the tiny travel bass I'm building, you'll still need a longer Y-axis than is standard.
The first version I made contain a mistake I (as a mechanical engineer) shouldn't have made, the row of holes on the far side should be slotted. Two cylindrical shafts in two round holes is overconstraint. I have corrected it in the attached PDF, but my physical version worked somewhat (getting the bridge fitted and removed was a bit of wrestling).
The bridge is fitted with two 6mm brass pins, and has an offset of 0.3mm wrt cheek of the bridge (half the thickness of the saw kerf). It is fitted with a few 8mm neo disc magnets of about 3mm thick. Those magnets hold the saw really well, I have tried to recess them slightly so the saw only runs against the wood but right now they are flush, no issues.
You do see the saw set chewing away at the bottom of the bridge.
I have attached the fingerboard with a bit of masking tape and superglue, and it is resting against a few reference pins, one against the bottom of the fingerboard, and two on the square edge of the fingerboard. There are several positions, so you can move pins if they are in the way.
but it should give enough clues on how to design one for your application.
