I have a festool router jig with a slide attachment. Totally acurate ajustment. It's pretty expensive so I used it to make housing jigs with 5/8 baltic birch ply. That way if a bearing fails or I have a brain fart I just make a new one. What I did was make all common widths but larger lengths. I have various inserts I use to bring the jig down to the various beam heights. That way I need less jigs and can make inserts with the miter saw. As far as uneven posts or cupped, crowned checked posts, the 5/8 is thick enough to stay pretty straight (flat). I use a mafell lo65 2200 router with a 1" bit with a template guide. That way I dont rely on the bearing much and the jig can be used for slightly undersized timber because the template guide will cut 7-3/8" wide on the 7-1/2" jig. I use a long shafted bit with a short depth of cut with more than one bearing on it
with this setup I can cut deep or shallow housings with the same bit/router.if I want to be sure to cut exactly the width of the jig instead of the 1/8" slim, I just set the router on another piece of birch ply with a hole for the template guide to drop in with the bearings sitting below the plywood slide so it bears on the housing jig edge. Then I just take that last slice off. I use the festool router slide to hog out the housing because it is thin enough for the template guide to drop through. Works great. On one plywood housing jig insert for each width size, I have a "plug" attatched to drop in the mortise so the housing is set exactly with the mortise without layout inaccuracies.
