I marked it out on the back and top side and with a handsaw cut to the back mark and top side mark every 1/2” or so, then chiselled out the timber between the cuts and tidied it up with razor rasps.
I was worrying about how to do it as I’ve not got a big bandsaw that I think some people use and I’ve done it before with a chainsaw (seriously) but it wasn’t easy or pleasant. I was going to get one of those planer/carving things for a 4” grinder but last night just tried how I mentioned and it was easy
Interesting approach. I was thinking a coping saw for hand cutting but I'm thinking it's going to be a long day sawing. Or the 40 grit on the orbital... messy but gentle in a way.