I maade such a unit after watching an internet video of a commercial U.S. made product. In fact, I made three in different sizes, as it involved buying a 6 metre length of steel C channel, for which I would have had no other use.
The idea is that you use the same jig to saw a board, and then switch to the router to joint the sawn edge. Couple of provisos: a. Need a smallish circular saw (trim saw), mounted on a base plate (for years I used one made fom a Plexiglass clipboard), such that the distance between the edge of the baseplate and the saw blade is 1 or 2 mm greater than the distance between the edhe of your router base and the router bit. BTW, that means a saw with the motor on the left, or else a saw with a worm drive - not available around here for ages. The latter is probably better for balance, but not critical, as the saw cut is just for rapid sizing. b. The hold-down board that comes with the jig may have to be replced, depending on the diamete of your router base. This is a simple matter: unscrew the old board from the top metal straightedge guide, replace with an oversize board, and trim to size first with the saw on its baseplate, then with your router. c Advisable to glue down strips of router anti-slip mat both on the metal base, and on underside of hold-down board, particularly if you will be working on laminate-covered boards. Otherwise you have to ratchet up the pressue on the clamps, to the point where the straight-edge may bow upwards, if it is long (my long unit can take full-size sheet goods). d. The final-width hold-down board must be wider than the base channel, otherwise you will route steel - noise, sparks, that sort of thing.
The use is simple: mark the board at both ends of the cut, slide it into the jig (under the hold-down board), align the marks with the edge of the hold-down board and clamp. Then cut with the saw, and trim with the router. Note that depending on the saw, you may be cutting from right to left, whereas the router will travel from left to right. Since both tools ride on the hold-down board, in theory the cuts should be square to the surface of the workpiece. The straightnes of the cuts depends on the straightnees of the metal straightedge (typically square-section steel tubing), but if the latte is not dead-on, all is not lost: the jig is double sided, so mating edges should be cut and trimmed on opposite sides of the jig to get matching edges.