I've done similar to Jim's pic. I used the makita chain mortiser clamped on the end to start the slot, then opened it up with a chainsaw. Not pretty, but got it done.
I'd look into a pipe style connection. If you can change the plate to a 1.25" dia pipe welded to the plate, you can drill a 1.5" hole. Then peg or bolt across as needed.
24" is still a long way to go. Seems overkill. Even in coastal regions with hurricane winds I think the longest post connection we needed was 18".
