I've tried a lot of them. I've used a board to mount, and roundover the baffle, I've cut holes that aren't always round to not look good in the end. I've used slots that always look good in the end.

The round-method I have always come back to involves gluing your port to the back of the board with poly-glue and clamping with a bar-bell weight. Drilling a hole through once cured, flush-trim to the port tube. Perfect every time! You can then roundover, chamfer, etc like usual. Details below:

Prep- (before box assembly)
Sand the port on the outside perimeter (not the end) beforehand with a staunch 80 grit or so. This breaks up the glazed/smooth surface, and allows the glue to grab hold. Mark your position on the opposing side of the board for easy drilling after glue has cured.

process-
Gorilla/Elmer's Ultimate/Epoxy are your best bets. Set the board flat. On your marked position, place your port and draw a circle around it. Remove port and apply heavy glue to the line, and enough to make sure the entire edge surface is covered, and also gluing to the sanded region. When you place the port, wiggle the edge to get it flat on the board and aligned with your circle. Once set in place, place a 5-10# weight on the other end of the port to prevent movement, and wait overnight to cure.

hole-
Drill with a bit large enough to fit your flush-trim bit in. Flush trim to the inner port wall, and then round or whatever you choose.

I pretty much do all my ports this way, and I like the results.
Wolf