MIMF

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.

Trying to add a PDF, but apparently, 271k is too big. See ZIP file for PDF, I have corrected it to have slotted holes.
I do understand a 530mm scale length is very weird but it should give enough clues on how to design one for your application.

Something I need to look into is compensation for the cutting kerf, the 6mm brass pins do fit pretty accurately, but there still is a tiny bit of play, should be possible to correct for the cutting kerf (if not for this jig, than for other templates to be made in the future).

Very clever, Jarno!

Made another one for the medium scale (32"-ish) fretless I have started to build. Scale length 810mm PDF attached. Note that if you want to cut this on a simple laser cutter, you'll need a Y-axis extension. Length of the part is 650mm.
Removed the locating pins, let's see if positioning the fretboard is problematic without them, but some of them were in the way of the sawguide positioning holes.

As mentioned above, no longer fitted the locating pins to rest the fingerboard against when sawing, I simply attached it using the painters tape and superglue trick, and that worked well.

LMII templates included a small tapered steel pin which eliminates all the slop.

LMII closed its doors this year, but it looks like Stew Mac may be taking over some parts and distribution.

It would be relatively easy to make a tapered pin in a lathe or even a hand drill by holding a file against the tip of an oversized pin.

It's important to note that this is poplar ply, because that is easy to cut on the diode laser, and that is a pretty weak material. So a cylindrical pin for alignment is fine, a wedging in a conical pin, not so much.

As mentioned, masking tape on the jig, masking tape on the fingerboard, bit of superglue in between, and that holds it just fine.