After a bit of trouble holding things down here is one attempt at a new method. It actually works quite well

a different method which works well if you do not push to hard

the feeler gauge works well to keep it all square

I use this square set to 118mm to mark the neck end block height

another square to trace that line around the block. this only works if the sides are square

a nice square edge

lines intersect to show the final block outline