I have a 110mm square laminated pine post.
I want to put a 45 degree bevel on the top of the post, around all four edges.
I don't have a mitre saw, but I do have a plunge router and a 45 degree chamfer bit.
Working on a scrap section of post, and chamfering from the sides of the post, not the top (that is, I rest the router base on the side of the post, bevel that edge of the top and then turn the post onto a different face) I've found that I can't keep the router level -- not enough of the router base is on the post.
Is there some jig or trick I should be using? Or is a plunge router an unsuitable tool for this job?

Just mark lines on the top and sides the same distance from the edges and plane to the lines. The only tricky thing is that you should skew the plane so it doesn't chip out the corner as it exits.
– SaSSafraS1232 Sep 18 '17 at 18:22