Never seen a splice with rivets before, the few I have seen are with bighead flat screws.
My hero mast builder a. doesn't have a portable welder, b. is not in favor of a butt weld to add on to cut down mast like Blaise had done. c. suggested the same inner sleve approach as mentioned above, cut a slice out of mast section, squeeze into mast, drill and tap.
Have an aluminum welder with a portable rig coming monday.
ED S. I have real concerns about positioning the mast step properly after removing it. My boat has 2 ears welded to the deck collar and had a bolt running thru the mast. If I get the base wrong, and use the bolt, the masthead 48 feet above the bolt will be off 6 or 7 x. I had very little rake, but the helm balanced beautifully as a sloop. Don't expect staysail to change balance much.
I have neither Malthurin's equipment or skill to make a riser. Would have to be designed and bought. I don't know, but my guess would be that it will cost near the foot of extrusion and take longer. kenyon already has 5280 extrusion. all they need do is slice me a piece.
Gonna take most of today off. Take the Admiral to a farmers market, then big lunch date at Home Depot. Think about what to do next.