I knocked up a slightly better mitre box using the first mitre box. Its also the same wood as the fret board bit so it's an interference fit in the channel with zero slop. I think I might cut another 'truing' slot. This ones not bad.. but its a little wide so the saw has more opportunity to wander.

So these pics are where I'm at. I dunno. It's a bit rough. Some of the goofs will disappear with radiusing, and the cuts are reasonably square. but I don't know that it'll cut the mustard as a finished board. I'll keep going to see if I can improve my technique a bit, I think I'm rushing.

I take it that it is fairly normal for the board to start flexing once it has a few cuts in it?

I'm also thinking I need to find a way to take some of the thickness out of the board before I start another one, I need to take off the radius plus around 2 mm to make it similar to my other guitars.