I'm looking to replace the old wooden round balusters on my stairs with square ones. The new square balusters have a post at bottom that will fit into the tenon OK but the holes in the banister are round so the square baluster is not going to fit. My initial plan was to simply miter the top of square baluster to angle of staircase and then screw or nail it in to banister. However, the holes in banister are pretty big and the square baluster will just about cover them so getting a nail into actual banister and not the hole will be difficult. I figured my other options are: 1. Square the holes so the baluster fits in. Con here is I'm not very experienced, probably don't have the proper tools to do this, and definitely don't want to have to remove the entire banister. 2. Fill in the hole i.e. cut a dowel and glue/screw it in and then nail the mitered baluster in. 3. Cut a small piece of dowel and screw it into top of square baluster so it fits in to banister hole. Hopefully I'd be able to fit the baluster in without have to remove banister or end piece of step. Am I on the right track with any of these or should I be doing this differently? Thanks/

I have 4 x 4 fence posts that are already in the ground, but the tops need to be shortened before the post caps go on. What is the easiest way to cut these squarely and accurately? Obviously my circular saw won't get through it with one pass. Thanks!