Rob Friedman wrote:so how can I trim the diameter down a touch? just filing ala peeling a carrot?
The 25.4 mm post should fit. Don't file the diameter as this will weaken the post. You perhaps did not notice that the clamp that cinches tight on the post when in place has an adjusting screw. You can loosen that screw and slightly widen the opening where the post sips in. It probably only needs that slight opening up to allow the 25.4mm post to fit properly. I also have 25.4mm post and it was easy to fit after i loosened the clamp's screw.
As far as shortening the post if it is a little too long, a regular hacksaw will do it fast in only a few minutes of sawing.