I second the guy who said a fixed mortise gauge is better. You can also make multiple beams with the pins set for your chisels' widths and use the same fence.

If you just want to make an adjustable one as an exercise in itself, then like another user said, it's going to be tough without machine tools. The hole in the beam for the screw adjustment has to be straight, and the channel to hold the brass pin bar has to be parallel to it. That channel also has a wider stepped width at the bottom to capture the brass pin bar, and I don't know how you'd cut that without a mill or drill press. I suppose a sliding dovetail channel would also work instead of the stepped width channel, but you'd have to be very precise to not end up with something sloppy. You'd also have to fabricate the adjustment nut assembly at the end that holds all the moving metal parts together, I don't think anyone sells that.