my experience was about 3 months ago, so my memory is a bit foggy. when I first tried to install the flat, I thought it was just plain-ol too wide. so I took about 1/8 inch off each outside edge of the flat. it was still a tight fit, but after several tries, putting it in at different angles, it finally dropped right into place. then I discovered that I had trimmed off too much, but a thick bead of epoxy putty on top and bottom of both edge joints covered up the slot caused by the excess trimming.
because of the way the sides of the hull are angled, I think there is just one “magic angle” that you need to position the flat in, relative to the floor of your workshop, at which it slides right in without much force being applied.
if you can’t find any angle at which it slides in, get out your block plane and shave a little bit off each outside edge and try again.