I will start a separate thread when I get to some construction but a little Jigging was todays Fusion for the first part of upgrading my 3018 Trash Router/Engraver in particular for PCB milling. I have been sitting on this Mobile phone repair clamp for way to long always with this in mind
Aliexpress Seller I usedPart of it's problem was always the flex in the supplied bed if you tried to clamp to it so I used to run heavy MDF then clamp to that which was a PITA. The base on this holder below is solid 6mm Aluminum so between it and the 5mm sacrificial MDF will lock the base up well.
One end of the four clamps has a 0.9mm deep rebate already cut into it and as they are 50mm wide plenty of area to provide even pressure on a PCB for routing. With adding a few extra holes and tapping them (eight) it will hold 10x20cm or 15x13cm boards fully supported and with some modified clamps on two edges a bit larger or more likely a different base but in reality larger boards are likely to be sent out and I would put up with the wait.
Shown is the sacrificial spoilboard that will get milled 'flatter' before use when attached to help get around some of the issue with PCB height variation. Also pre milling probing of the PCB surface via Flatcam. Spoilboards will get the Laser cutting treatment because I have one so nice and quick and accurate each time for hole locations.
Brushless motor and improved Z axis are the next and biggest bits to design.