Whether because of my progressive lenses or inexperience, I was having trouble satisfying myself with winding sticks that the tops of the side boards were in the same plane. So, as a doublecheck, I stretched string tightly across the diagonals and was relieved to find that they intersected in the middle without deflection. The last thing I want to do is remove a lot of material from the top, so I think this is a critical step. Not sure if you can see the string:
As you can see from the shavings, I did have to remove some material to get the side boards in the same plane.
I made the transverse bearers today, but didn't get them installed. I settled on seven:
I did some testing with a dead blow hammer and a block. It seems really solid.